Please see bug trace.
rEWMV/modules/ext.wikimediaEvents.searchSuggest.js seems to be involved.
There has been no deployment yesterday, but events are triggered by a 1/1000 ratio and might have been sleeping a couple of weeks now.
The reporting user has a very slow network connection, which may make her loosing a race condition, while others got the module in place.
Watched 2015-10-16 11:07+0200.