448: I am not allowed to give NPC's potions of True Strike. This applies whether I am a PC or DM.
449: I am not allowed to give NPC's arrows or bolts of Harm (edit: Or Empathic Transfer: Hostile). Again, whether I am a PC or a DM.
450: I am not allowed to give multiple NPC's massive damage first strike abilities at the same time. If I do, I am not allowed to have them all use those abilities in the same round.

451) I am not allowed to wear boots of striding on a monk
452) Nor is he allowed boots of Skate
453) Nor is he allowed the Run feat
454) I am not allowed to cross the Anaroch desert on Faerun in under a day, even if my level 18 monk's movement, equipment, and feats allow it

455) In an Epic campaign, I am not allowed to use Leadership to have an army of Psychoanalysts.
456) I am not allowed to use said army to convince all the orcs of the land that they suffer from an Oedipus complex
457) I am not allowed to civilize said orcs to the point that they're more advanced than humans

458) Not allowed to bribe orcs with nachos.

459) Tieflings are not allergic to mystical fairy pollen.

500) Yes, I must roll for an alignment change/sanity roll/whatever when the GM discovers the mystical fairy plants that I took the fairy pollen from were plotting with me to take over the world.

501) My bard is not to challenge anyone in the 9 Hells to a fiddling contest ever again.

From back in my Army days:

502) No, I cannot turn the Officers in my chain of command because they are evil.
503) The privates in the squad do as they are ordered because of the chain of command, not because the sergeant has charmed them.
504) The chain of command is not something you beat some one with till they figure out who is in charge.

505) I am not allowed to split the mission reward with the target in exchange for a promised to stop misbehaving enforced by a binding ritual.
506) Even if I do that, I am then not allowed to negotiate a restitution for the years the target has been "kept down by The Man."
506a) Especially if I or the party was "The Man" in question.
507) I am not allowed to venture into the monster's territory, after having been at least partially paid to exterminate said monsters, and then throw in with their cause for an additional up-front fee. I think it was the "up-front fee" part that really ticked the DM off.

Ah, good times...

508) I am not allowed to set other party members on fire "as a science experiment."
509) Ditto for holding them underwater.
510) Ditto again for dropping them off cliffs.
511) It seems that, no matter my character's Intelligence score, I am supposed to assume a certain number of truths of how the physical world works that do not need to be tested.
511a) Certainly not more than once, anyway.

From my college roommate...

512) I am not allowed to drink alcohol while playing D&D.
513) I am especially not allowed to drink alcohol while playing D&D if we are playing by candlelight.
514) If I ignore the above two rules and set my character sheet on fire again, I am not allowed to try and flick the flaming paper at the DM, yelling "Fireball, you son of a bitch!"
515) If all of the above happen again, it will be my turn to explain it to the RA.

More as I collect them from friends and sort-of-friends...

516) I am not allowed to instigate an in-person flame war among my fellow players on any of the following topics: the un/importance of proper grammar, special editions of Star Wars, the best Doctor, whether The Princess Bride is better/worse than Army of Darkness, or--especially--editions of D&D.

517) I am especially not allowed to take advantage of my fellow players' involvement in such a debate to pass notes to the DM unnoticed by them.

518) I am especially especially not allowed to take advantage of my DM's involvement in such a debate to make changes to my charsheet/his notes unnoticed.

519) ...and Heaven help me if I try to take advantage of such a distraction-filled environment to snag the last Dr Pepper unnoticed.

The problem with 516 is that you cannot start an argument over the best Doctor, since it is clearly David Tennant.

No, he was a bit too shouty. Matt Smith plays an excellent balance of eccentricity and a man who's seen many things, including many dark and disturbing things.

But we should keep such off topic discussions out of here.