This is related to the OOUI component that wraps the central search bar on Special:Search: this component is what turns the plain html text input box for search into an autocomplete.
Seen on various search pages
https://logstash.wikimedia.org/app/dashboards#/doc/logstash-*/logstash-2021.04.20?id=t5c58HgBGiM4niWIb8GP
message
Error: Widget not found
trace
at OO.ui.Element.static.unsafeInfuse URL1:183:48 at OO.ui.Element.static.infuse URL1:182:545 at OO.ui.infuse URL1:179:29 at URL1:792:596 at mightThrow URL1:53:141 at resolve/</process< URL1:53:800 URL1: https://ro.wikipedia.org/w/load.php?lang=ro&modules=Spinner%2Cjquery%2Coojs%2Coojs-router%2Coojs-ui-core%2Coojs-ui-widgets%2Csite%7Cext.advancedSearch.elements%2Cinit%2Cstyles%7Cext.centralNotice.geoIP%7Cext.centralauth.centralautologin%7Cext.cirrus.serp%7Cext.cx.eventlogging.campaigns%7Cext.dismissableSiteNotice%2CeventLogging%2CnavigationTiming%2Cpopups%2CwikimediaEvents%7Cext.tmh.OgvJsSupport%7Cext.uls.common%2Ccompactlinks%2Cinterface%2Cpreferences%2Cwebfonts%7Cjquery.client%2Ccookie%2CembedPlayer%2ChighlightText%2CloadingSpinner%2CmwEmbedUtil%2Csuggestions%2CtextSelection%2CtriggerQueueCallback%7Cjquery.uls.data%7Cmediawiki.String%2CTitle%2CUri%2Capi%2Cbase%2Ccldr%2Ccookie%2Cexperiments%2CjqueryMsg%2Clanguage%2Crouter%2CsearchSuggest%2Cstorage%2Cuser%2Cutil%2Cwidgets%7Cmediawiki.editfont.styles%7Cmediawiki.libs.pluralruleparser%7Cmediawiki.page.ready%7Cmediawiki.special.search%7Cmediawiki.widgets.SearchInputWidget%7Cmw.EmbedPlayer.loader%7Cmw.MediaWikiPlayer.loader%7Cmw.MwEmbedSupport%2CPopUpMediaTransform%7Cmw.MwEmbedSupport.style%7Cmw.PopUpMediaTransform.styles%7Cmw.TimedText.loader%7Coojs-ui-widgets.icons%7Coojs-ui.styles.icons-content%2Cicons-interactions%2Cicons-layout%7Cskins.vector.legacy.js%7Cuser.defaults&skin=vector&version=ieqv3
Not sure how to replicate. Perhaps the search uses OOUI server side rendering only on a small amount of page views?
It looks like 20-112 hits in the last day, so this doesn't seem to be coming up a lot