I was recently editing a question that used the qtonpi tag. When I had finished my edit I selected Save Edits, only to get this:
Obviously, I found this a bit strange, since qtonpi is an actual defined tag and has been used multiple times - and there is no 'pi' tag on this question.
I removed the offending tag and I was able to submit. Not sure if this is a bug in the tag detection algorithm thingy or what. Just thought I'd post about it.