The ti-99 tag appears to be unusable. Its page exists, as does the ti99 tag’s page, but its info page doesn’t. Trying to assign it to a question doesn’t work; it can be entered, and is recognised as a tag:
but trying to save the tags produces an error message:
“You are attempting to create the tag [ti-99]; however the tag [ti-99] already exists! If you think this new tag should be allowed, raise it on meta”
I imagine this is related to Should the ti99 tag be changed to ti-99? and can hopefully be fixed by a moderator.