Left 4 Dead Glitches

Skip major portion of No Mercy: The Apartments
This is initiated at the very beginning of the first chapter in the first Campaign, No Mercy. Directly behind you at the start of the game is a ledge. Across the alley, on the building opposite, is an air-conditioning vent that can be reached by jumping off the ledge and landing on it. This will consumer roughly around half your character's health, but will skip a large portion of the gameplay that includes moving through the Apartments to reach the entrance.