It just happened to me when asking a question.
I wanted to add the built-in functions tag to my question. When I start typing it, this appears as an option, although I cannot choose it. Still, when I finish typing it, the words transform themselves into the built-in-functions tag. However when I post the question, this tag turns into the built-in-symbols tag by some reason. I believe this must be considered as a bug, no?
Pi
, many option names, etc.), all built-in functions are built-in symbols. So it's a distinction without much of a difference. I'll edit the tag wiki to clarify. – Michael E2 Aug 21 '16 at 12:01