Every other chat room works as expected.
There was some failure earlier with regards to the list of all Q&A sites that chat maintains. I'm still digging for what exactly happened, but I'm not sure I'll be able to find the precise cause.
Whatever the issue was, it caused chat to not know about several sites. This caused these sites to be dropped from a user's account list, since we cross-check the user's accounts with the site list. The user's parent account would then be set to one on a site that we still do know about.
From now on, when this situation occurs (i.e. the API gives us a user account for a site we don't know about), we won't just swallow this fact (and account), but take it as a hint that something is ill with our data and a) force a reload of the sites data, and b) bail out and rather throw an exception than taking away a user's account.
I have also fixed up the accounts for moderators affected by this (since for them it's obviously important that the system knows they have a moderator account somewhere).