Left 4 Dead Glitches

Reload whilst Meleeing
This trick has only been proven to work with either of the two shotguns, due to the fact that reloading cause the bullets to stack, and isn't reloaded with one single clip.
To perform the glitch, simply press the melee button as soon as you choose to reload. You character will initiate the melee action, which will knock the infected back, therefore cancelling the reload animation. Despite this, your gun will still continue reloading, making it much easier to keep yourself safe whilst reloading the shotgun.