Two out of many repos (projects) mysteriously missing

A strange thing was reported to me today – a couple of projects hosted on a server with a whole bunch of other projects went missing somehow. The developer was about to make some changes on a project (repo) which hadn’t been touched in a month, according to the metadata, which remains. There is enough evidence left of this repo that the fsck operation returns an error about the repo not existing.

I am not sure how this occurred. It didn’t happen today; the oldest backup of the instance (not all that old) reports it missing also.

Luckily, the most current versions of the two affected repos were checked out to some developers’ machines, so we should be able to rescue the code, and possibly also the history, if I can figure out how.

But I found this to be very odd. Has anyone else ever seen a similar thing?

(and as the sidebar says, it is very similar to that issue 5372 but that ticket contains no useful data)