Ahh! It's the ! That's good to know. I usually enter in raw html. But I loaded the gui to speed up nice formatting (inserting lists, indenting etc.) I did that just before 'fixing' the Tex breakage, so I thought it was "Tex" itself. Now I know to just edit the raw html when I see the [?].
Naturally, since I loaded Friday, I notice other 'infelicities', and then need to find solutions. Glad I know the issue here.
Now on to the 'infelicity' associated with trying to include Tex in 'calculated questions'. (There seems to be a problem with both using { } as delimiters. Right now my solution is "don't use Tex" in calculated questions!)