Rule Zero: All other rules can be changed by the GM.
I appreciate that there is a massive amount of work that goes into being a GM and the last thing that any GM wants or deserves is to have his players constantly questioning everything they do. However, a group I am currently playing with has been having a huge problem with a GM who has been using rule 0 to deny virtually every skill check and give all his custom enemies powers that have either no save or unbeatable DCs. Then, whenever he realizes he has made the fight unwinnable he brings in one of his level 60+ GMPCs to instantly win the fight, making us feel even more helpless.
We try to hold our tongues and not be the dreaded Rules Lawyer, but he keeps pulling out stuff so ridiculous that we have to question it… and every time we do we find that he is either misunderstanding or completely disregarding the rules. When this happens he always jumps immediately to rule 0. He probably cited rule 0 around 6 times in our last session (and we only had one combat encounter).