What kind of error is this? From what service and program/runtime? Is this server-side EventGate validation?

If this is related to searchTerm than it is this code:
https://gerrit.wikimedia.org/g/mediawiki/extensions/WikimediaEvents/+/72cdfdc68a5a4cc652ae3f81ab5bb53d58fc46dc/modules/ext.wikimediaEvents/searchSatisfaction.js#547

		params = {
			query: mw.config.get( 'searchTerm' ),
			hitsReturned: $( '.results-info' ).data( 'mw-num-results-total' ),
			extraParams: JSON.stringify( serpExtras )
		};

Ending in the SearchSatisfaction event

The mw.config part is set in https://gerrit.wikimedia.org/g/mediawiki/core/+/78c1095d35bb3faf33786eadfe1717ccd141bf0e/includes/specials/SpecialSearch.php#666
but SpecialSearch::setupPage is documented to get a string. The value passed to it is running through str_replace, but that supports arrays. The string is coming from $request->getText( 'search' ) which looks safe even for array parameters on GET requests.

Thanks, @Umherirrender.

There are other code paths that set the query property of the SearchSatisfaction events:

1. https://gerrit.wikimedia.org/g/mediawiki/extensions/WikimediaEvents/+/72cdfdc68a5a4cc652ae3f81ab5bb53d58fc46dc/modules/ext.wikimediaEvents/searchSatisfaction.js#708 for action="searchResultPage" events
2. https://gerrit.wikimedia.org/g/mediawiki/extensions/WikimediaEvents/+/72cdfdc68a5a4cc652ae3f81ab5bb53d58fc46dc/modules/ext.wikimediaEvents/searchSatisfaction.js#508 for action="visitPage" events

Looking through a sample of EventGate validation errors, the events are action="visitPage" events, i.e. it's #2 that's setting the invalid property to the event. Unless there's a (serious?) bug with mw.Uri, then I'm inclined to think that there's two search query parameters in the referrer.

I noticed that elements of the array are the same string, i.e. the referrer looks like query=foo&query=foo. I tried searching for "foo" on wikidatawiki and was sent to a SERP with a URL like https://www.wikidata.org/w/index.php?search=foo&search=foo&title=Special%3ASearch. I don't see this on enwiki, for example.

Update

There's two fields named "search" in the Wikidata search box:

Wikidata does provide its own implementation of search. There is also evidence of a gadget that completely replaces the form that may be responsible.

Change 670815 had a related patch set uploaded (by Phuedx; owner: Phuedx):
[mediawiki/extensions/WikimediaEvents@master] searchSatisfaction: Handle edge case in referrer URLs

https://gerrit.wikimedia.org/r/670815

In T276474#6902366, @Jdlrobson wrote:

Wikidata does provide its own implementation of search. There is also evidence of a gadget that completely replaces the form that may be responsible.

After writing the above patch, I found that the "search" hidden input on wikidatawiki is added by https://gerrit.wikimedia.org/g/mediawiki/extensions/Wikibase/+/87e2b58c8b426c44b84d29beb4526908ce349d71/repo/resources/wikibase.ui.entitysearch.js#15. I suspect this can be removed now since it was added to work around a bug in Firefox (see T60467: Wrong smart keyword on Wikidata).

Edit

I've just tested the Firefox feature on enwiki with JavaScript disabled and it appears to be working.

Change 670815 merged by jenkins-bot:
[mediawiki/extensions/WikimediaEvents@master] searchSatisfaction: Handle edge case in referrer URLs

https://gerrit.wikimedia.org/r/670815

Test Result - Beta

Status: ✅ PASS
Environment: beta
OS: macOS Big Sur
Browser: Chrome
Device: MBP
Emulated Device: NA

Test Artifact(s):

QA Steps

Scenario 1

1. Navigate to https://wikidata.beta.wmflabs.org/
2. Disable V2 of Vector, if you're using it
3. Run the following in your browser's console:

localStorage.setItem( 'wmE-sS--sessionId', 'T276474' );
localStorage.setItem( 'wmE-sS--sessionIdEndTime', Date.now() + 24 * 60 * 60 * 1000 );

4. Refresh the page
5. Search for "foo" using the search widget in the top right-hand corner
6. Wait for ~30s
7. ✅ AC1: Observe that a SearchSatisfaction event has been logged with action=searchResultPage and query=foo

Scenario 2

1. Navigate to https://en.wikipedia.beta.wmflabs.org/
2. Disable V2 of Vector, if you're using it
3. Run the following in your browser's console:

localStorage.setItem( 'wmE-sS--sessionId', 'T276474' );
localStorage.setItem( 'wmE-sS--sessionIdEndTime', Date.now() + 24 * 60 * 60 * 1000 );

4. Refresh the page
5. Search for "bar" using the search widget in the top right-hand corner
6. Wait for ~30s
7. ✅ AC2: Observe that a SearchSatisfaction event has been logged with action=searchResultPage and query=bar