User A posts a question.
User B edits it.
Mod C adds a mod-only tag.
User A doesn't like user B's edit, so clicks "edit" next to the original revision, edits in the same tag mod C had added, and tries to save.
He'll be told he can't, as the tag is mod-only.
This appears to be a bug: the software should allow an edit that keeps the current tags.