ITT we discuss Game Breakage that we are fond of.
Example: Today, I was playing Fable on my computer. Normally, when you save in Fable, the game saves everything; however, if you save in the middle of a quest, the game "Hero Saves", instead of "World Saves"; in other words, it records everything but your progress in your current quest. One of the quests, a bit less than halfway through the game, is the Arena quest; 10 rounds, kill stuff, get moneys.
Hero Save on round 6 or so, reload, continue. The Arena is a killing field and makes it absurdly easy to rack up amazing amounts of experience, especially taking into account the other massive abuse Fable contains: the Physical Shield spell.
The more hits you chain together without getting hit, the higher your Combat Multiplier goes. The Multiplier does just that; it multiplies all experience you receive by itself. In the Arena, it is easy to get the multiplier up to the 30s or 40s. For every time you get hit, though, the multiplier goes down to the last number divisible by 5...unless you have Physical Shield active. While active, you take hits to your mana instead of health, and getting hit neither makes you stagger nor subtracts from your multiplier.
You can get - literally - hundreds of thousands of experience in the Arena in half an hour, enough to max out the important skills and with a bunch left over.
Any other game breakers you can think of? Share 'em!