It was suggested that I suggest this suggestion. Suggestively.
We've had two cases now where retaggers got confused because they tried to change a new tag to a more common tag and nothing happened. It turned out tag synonyms were silently renaming them. I think silent renaming makes sense for an initial question post (the user probably won't even notice the tag changed), but for an edit that retags a question it seems to the user like something went wrong and the edit didn't go through. Should there be a warning that a tag was remapped, but only for retagging edits?