Baroness said:
When the GM sticks an NPC with the group and the NPC receives more attention than the rest, is stronger and is the solution to every problem, it's time to leave and let the GM play with himself.(not that way

)
Ah, the pet NPC. I've seen all too many of these. I think about 25% of the campaigns I play in have one.
Horror Story 1: One GM had a pet NPC of the week. We'd get togeather, and instead of playing the adventure, it would be "Look at my new character.. I mean this NPC that will help you! FOr the next four hours, you will encounter him, talk to him, and be saved by him." That GM, more than once, made barbarian/monk combos (a halfling barkeep barbarian/monk with a heavy Irish accent) and at least once made the same class combo as a PC, but gave the NPC better equipment and better stats. Fortunately, we let the GM know about the problem, and he stopped. A happy ending.
Horror Story 2: This was a 1 on 1 campaign from way back in the day. The GM introduced 6 NPCs to accompany me, each one from his stories that he wrote when he was in Jr.High. In the first fight, after round 1, he skipped me in combat. I sat there and watched as, for 5 rounds of combat, he had his NPCs fight the monsters, and win, and never realized that I had yet to act.
Agent Oracle, how about one of the signs being "Just to let you know, I've house-ruled the system. Oh, but I'll not tell you what rules I changed. You'll find out once the rule comes up, and you start using it as normal. I'll throw a temper tantrum, then a week later say that I had my own version of the rules, but I'll still never tell you what they were, or what exact situation used my house rules instead of the RAW."
Is that too specific to my own experences? That specific GM could lead to page after page after page of ranting..