If you have a tag "X", which is a synonym to the master tag "Y", and then you try to merge "Y" into "X" (with the "These two tags are synonyms" box checked), the merge preview succeeds, but the merge itself fails and sends you to a generic "Oops! Something Bad Happened!" error page.
It would be better if either the preview would tell me that I'm doing something stupid and stop me from trying to go ahead, or at least if a specific error message could be thrown rather than the generic "Oops!" page.
(Why would I even do something stupid like this? Because I forgot that you can just click "swap" on /tags/synonyms to do this instead.)