Merges (of tags, users or questions) are, if not impossible to undo, then at least very hard, and requiring developer intervention.
Please keep a log of merges, at least tag merges, in some easily-exploitable form. Bad tag merges (where the tags weren't quite synonyms after all) happen now and then, and having a list of retagged questions would be a great help.
I don't see any issue with making tag merge logs public (buried somewhere in the UI, presumably in the synonym dashboard). User merges, if logged, would of course need to be kept under wraps.