Inspired by Possible invalid merge of [cpp] into [c++].
Currently once a tag synonym is accepted by being upvoted by 3 people with 2000 or more reputation, the tag with the lowest count is permanently deleted and all questions tagged with it will have the tag replaced with the synonym. This cannot be undone.
Quoting waffle's post in the thread linked above:
Fixing this mess is a real cluster.
Our merge process does not have an audit trail, it does a pure global replace everywhere, there is no undo.
I'm extremely concerned about this. Someone can just create a couple of sock puppets account, get to 2000 reputation with each, create proposals to make all the popular tags synonyms of C# (for example c, c++, java, etc), upvote himself or herself with the sock puppet accounts, and the tags will be permanently* gone.
* Of course they can be recovered from the data dump or backups, but this will involve a lot of development time and headaches.