I'm lucky I guess, DS has tended to do very well on past papers etc so has been quite relaxed about them. But he has been bemused by the fact teachers /TAs have been going round giving a lot of help with answering the questions!
In two cases he said the TA definitely got it wrong as well. (his friend from another school, also very able was over and agreed with DS about the question and the correct answer )
Surely they are pointless if this happens in some schools (DS friend didn't see it happen in his school to be fair)