I'm not necessarily opposed to this, but how does a mis-synonym "destroy data irrevocably"? You check all the posts tagged sql since the synonym was created and retag the ones that should be sequel. It's tag merges that really screw stuff up, since you now need to check every post tagged sql, but that's why merges can only be done by moderators
Edit: I just read waffles' post:
what more now there are no questions tagged sequel to be found.
This wouldn't happen from a synonym creation; it sounds like Bill merged the tags. Are you asking for a way to undo tag merges instead of tag synonyms?