Martin, thanks for suggesting this very useful feature. Since it's enabled, I had it maybe 5-6 times: precisely in a post I click "edit tags" and type, for instance, "gt", and what happened is that the cursor is put automatically at the end of the list of the question's tags, only after I typed "g". This resulted in changing, say, "set-theory" to "gset-theory". But my eyes weren't on the tag set-theory so it was unnoticed; I just retyped "gt" instead of "t", and in this way I added the new tag while adding a typo in another tag. Now this results in the warning and it's great. — YCor yesterday