Seems to have been introduced in wmf30. This is currently the top client error.
449,290 errors in the last 7 days in Wikimedia Commons only (This is extremely high error rate - 2nd-highest error rate is only 14,681 errors in same period)
Stack traces point to this code:
https://gerrit.wikimedia.org/g/mediawiki/extensions/MediaSearch/+/9c32c81792dbacb89bda6676eb33c570a0090072/resources/components/App.vue#369
stack trace for event is undefined
at reportLoadTiming https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:305:345 at created https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:305:443 at Vt https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:406:797 at Bt https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:406:869 at Bt https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:406:968 at Ir https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:434:869 at resources/lib/vue/vue.js/yi/< https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:467:711 at yi https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:468:188 at resources/lib/vue/vue.js/di/s< https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:465:847 at di https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:465:853 at M https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:450:354 at P https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:450:209 at m https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:447:233 at X https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:457:418 at mount https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:442:678 at resources/lib/vue/vue.js/Xc/t.mount https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:487:870 at resources/init.js/< https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:259:653 at resources/init.js https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%7Cext.centralauth.centralautologin%7Cext.eventLogging%2CwikimediaEvents%7Cmediawiki.base%7Cmediawiki.page.ready&skin=vector&version=5vcka:259:673 at runScript https://commons.wikimedia.org/w/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector:11:831 at execute https://commons.wikimedia.org/w/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector:13:96 at doPropagation https://commons.wikimedia.org/w/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector:5:943
stack trace for Cannot read property 'loadEventEnd' of undefined
at reportLoadTiming https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:305:389 at Proxy.created https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:305:443 at Vt https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:406:797 at Bt https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:406:867 at Bt https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:406:968 at Ir https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:434:867 at https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:467:711 at yi https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:468:188 at https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:465:845 at di https://commons.m.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue&skin=minerva&version=7kkzg:465:853
stack trace for undefined is not an object (evaluating 'event.loadEventEnd')
at reportLoadTiming https://commons.m.wikimedia.org/w/load.php:305:388 at created https://commons.m.wikimedia.org/w/load.php:305:459 at Vt https://commons.m.wikimedia.org/w/load.php:406:798 at Bt https://commons.m.wikimedia.org/w/load.php:406:869 at Bt https://commons.m.wikimedia.org/w/load.php:406:970 at Ir https://commons.m.wikimedia.org/w/load.php:434:869 at https://commons.m.wikimedia.org/w/load.php:467:713 at yi https://commons.m.wikimedia.org/w/load.php:468:188 at https://commons.m.wikimedia.org/w/load.php:465:847 at di https://commons.m.wikimedia.org/w/load.php:465:853 at M https://commons.m.wikimedia.org/w/load.php:450:356 at P https://commons.m.wikimedia.org/w/load.php:450:210 at m https://commons.m.wikimedia.org/w/load.php:447:234 at X https://commons.m.wikimedia.org/w/load.php:457:419 at mount https://commons.m.wikimedia.org/w/load.php:442:679 at https://commons.m.wikimedia.org/w/load.php:487:870 at https://commons.m.wikimedia.org/w/load.php:259:658 at resources/init.js https://commons.m.wikimedia.org/w/load.php:259:673 at runScript https://commons.m.wikimedia.org/w/load.php:11:831 at execute https://commons.m.wikimedia.org/w/load.php:13:105 at doPropagation https://commons.m.wikimedia.org/w/load.php:5:943 at https://commons.m.wikimedia.org/w/load.php:22:737
can't access property "loadEventEnd", event is undefined
at reportLoadTiming https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:305:383 at created https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:305:443 at Vt https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:406:797 at Bt https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:406:869 at Bt https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:406:968 at Ir https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:434:869 at resources/lib/vue/vue.js/yi/< https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:467:711 at yi https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:468:188 at resources/lib/vue/vue.js/di/s< https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:465:847 at di https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:465:853 at M https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:450:354 at P https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:450:209 at m https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:447:233 at X https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:457:418 at mount https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:442:678 at resources/lib/vue/vue.js/Xc/t.mount https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:487:870 at resources/init.js/< https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:259:653 at resources/init.js https://commons.wikimedia.org/w/load.php?lang=en&modules=%40wikimedia%2Fcodex%2Cjquery%2Cmediasearch%2Cvue%2Cvuex%7Cext.centralauth.ForeignApi%7Cext.wikimediaEvents.specialPages%7Cmediawiki.ForeignApi%7Cmediawiki.ForeignApi.core&skin=vector&version=ygfdr:259:673 at runScript https://commons.wikimedia.org/w/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector:11:831 at execute https://commons.wikimedia.org/w/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector:13:96 at doPropagation https://commons.wikimedia.org/w/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector:5:943
