Created attachment 16195
Screenshot by Mutante

Attached:

Change 154138 had a related patch set uploaded by Legoktm:
Normalize key before creating a Skin object

https://gerrit.wikimedia.org/r/154138

Change 154139 had a related patch set uploaded by Legoktm:
Normalize key before creating a Skin object

https://gerrit.wikimedia.org/r/154139

Change 154138 merged by Bartosz Dziewoński:
Normalize key before creating a Skin object

https://gerrit.wikimedia.org/r/154138

The above patch, when deployed, should fix the issue for logged-in users who haven't changed their preferences in a very long time and had some values set that are currently invalid – these weren't being normalized properly and the users would get the fallback skin instead of the default skin, even thought the default skin was available.

Mutante said he saw that when logged-out, though, and we still have no idea what could possibly cause that (other than managing to hit the site at some split-second during deployment when not all files were there properly?). Bug 69284 may or may not be relevant.

Change 154139 merged by jenkins-bot:
Normalize key before creating a Skin object

https://gerrit.wikimedia.org/r/154139

Should be fixed now.