Edena's tale of the constantly-doubling experience awards reminds me of one of my favourite mistakes in AD&D, made when I was but fourteen:
One of my players of similar age played a Mage named (uncreatively but surprisingly-appropriately) Merlin, and made liberal use of a necklace he had acquired around 9th or 10th level which contained five charges of a homebrewed 9th Level spell called Michelle's Chaos Wind (similar to a Finger of Death capable of effecting multiple targets). With this he killed more than one quasi-/demi-deity (unlucky saves on their part) and I, lacking official XP values for such things, decided to go the rout of impressing said player with progressively ludicrously high numbers.
Looking back at the character sheet a few years later, while I am not sure if I actually awarded a trillion or more XP, I calculated from what it looked like on that oft-erased area of the sheet and determined that he could be of around 6,053,008th level. I am glad that while we were playing I ruled that noone was available who could train Merlin beyond 20th level, but I would be very interested in knowing how Gary would handle a campaign with PCs of seven-digit level (I suspect remarkably well, but I am curious as to what kinds of challenges would be faced by said PCs).