I just attempted a tag merge as per this tag synonym request, merging all instances of sso with single-sign-on and creating a synonym reference mapping the former to the latter, but it ended in a cat-astrophic failure. A few minutes later I tried it again, with the same result. I decided to investigate the problem, and tagged the question status-planned in the meantime.
As a Plan B, I thought of creating the synonym first, then performing the merge separately (although I don't even know if that is necessary by the time the synonym is made).
So, I visited the tag info page for sso, only to notice that there's a proposed synonym mapping single-sign-on to it (which apparently was made back in December 2010 before going unnoticed altogether!). Since I'd rather have the abbreviation pointing to the full name, I swapped the synonym proposal before approving it.
So far, so good, but this made me realize that the merge failure could have been caused by the existing synonym proposal being in the opposite direction of what I was trying to do in the merge. To test this hypothesis, I went and attempted the merge once more (again, not sure if it was necessary at all). And lo and behold, it worked.
I've managed to resolve the issue, so I've marked the linked question status-completed, but I'm guessing something needs to be fixed on the backend, hence this bug report.