Another to add to the OP:
I posted a thread asking where GAF's ToS were (had searched the FAQ forum) and got banned for a week. Yes, it was a stupid question, but I had honestly always glossed over that blue bar at the top of the screen (something explained in the thread pre-ban). My ban reason was "sheer stupidity".
So, yes, being stupid and/or unobservant is bannable.
There are two things that we've been told were bannable that aren't enforced though:
-Not reading the first post of a thread
This seems not to apply anymore...was enforced in a single thread, IIRC.
-FHUTA
When the ban-worthy memes thread was put up in the FAQ, it originally included this, stating that it was offensive and sexist. It has since been removed, and I don't believe it was ever enforced.