I'll admit it: I'm not above exploiting a game if I feel like it adds to the experience. In the case of Left 4 Dead 2, when I ran into some players who taught me a trick to lagging out the game, I was happy to play along.
The trick was you had to quickly swap between dual pistols and some kind of melee weapon. Eventually the technique would start producing duplicate pistols and, if you kept it up long enough, you'd spam the floor with them. The game would subsequently lag out, meaning you could easily avoid enemies' paths instead of bothering yourself with killing them. Why would you do this? To run through each level on expert mode, of course! That may sound like it kills the challenge and purpose of the game, but it was the tank battles and confrontations with Left 4 Dead 2's other special enemies that were all the more entertaining because of it.
I distinctly remember the combat-dance my teammates and I had to partake in against the tank, timing hits as best we could, taking the delay of its actions into account. Eventually you'd start to really learn its movements, and it felt like an entirely new boss with just the right amount of unpredictability.
So, what are your exploit stories?