During the bit with the three porn bimbos, Howard asked the question, "In which country was Osama Bin Laden killed IN." Robin immediately reacted to the mistake, and even when it was pointed out to him that he had made a grammatical error, Wiggy didn't seem to understand what Robin was alluding to.
She had this attitude about her which was priceless, as if she had a thought balloon over her head which said, "Howard you're such a dumbass." He tried to dismiss it and was probably annoyed at it being pointed out.
Quite often I've thought Howard IS as dumb as a box of rocks. His debating skills are poor, he often can't remember things he's said or if he's had certain guests on the show or not. He's made a career out of simply being juvenile and being devoid any form of intellectual wit. At this point he's either grown complacent and sloppy on air, or he's getting old and senile. Whatever the reason, to me his on air persona isn't particuarly likeable anymore, and hasn't been for a while now.