Change 291040 had a related patch set uploaded (by Legoktm):
TitleParser: In formatTitle(), don't throw exceptions on bad namespaces

https://gerrit.wikimedia.org/r/291040

Mentioned in SAL [2016-05-26T20:38:05Z] <twentyafterfour@tin> Synchronized php-1.28.0-wmf.3/includes/specials/SpecialSearch.php: deploy hotfix for itwiki search T136356 (duration: 00m 23s)

itwiki search is working again after deploying rMW49360d0eae61.

That hotfix is not on master so we need to find a better solution before next Thursday.

Change 291099 had a related patch set uploaded (by Legoktm):
TitleParser: In formatTitle(), don't throw exceptions on bad namespaces

https://gerrit.wikimedia.org/r/291099

Change 291040 merged by jenkins-bot:
TitleParser: In formatTitle(), don't throw exceptions on bad namespaces

https://gerrit.wikimedia.org/r/291040

Change 291099 merged by Legoktm:
TitleParser: In formatTitle(), don't throw exceptions on bad namespaces

https://gerrit.wikimedia.org/r/291099

Mentioned in SAL [2016-05-26T21:43:48Z] <legoktm@tin> Synchronized php-1.28.0-wmf.3/includes/title/MediaWikiTitleCodec.php: TitleParser: In formatTitle(), don't throw exceptions on bad namespaces - T136352, T136356 (duration: 00m 28s)

The same thing causes issues for us was also causing issues elsewhere. https://gerrit.wikimedia.org/r/291099 has been deployed to band aid other things, so we can probably undo (or just not pull into master) our fix above.

This doesn't solve the unknown namespace ID problem, it just hides it.

The immediate problem is fixed. The more general problem will be fixed later when we being to work on cross-project indices.

Thanks everyone for fixing the immediate problem and unblocking the deployment.

This is no longer a blocker for wmf.4 since rMWa22ab0eab02c: TitleParser: In formatTitle(), don't throw exceptions on bad namespaces made it into master.