Double Melee

The double melee was a glitch that allowed survivors to melee two times in extremely quick sucession. It was fixed in the January 23, 2009 update.

How to Do the Double Melee
By pressing the melee button and the change weapon button in rapid sucession, the survivor will melee extremely quickly, and does not have to stop. This is extremely effective when being surrounded by a horde, or when ammo is scarce as the double melee will kill many infected rather quickly. This tactic is also extremely effective when escaping to the rescue vehicle during the finales.

This tactic is also effective in versus mode as it can easily destroy someone wanting to pounce as a Hunter.

Effectiveness
The double-melee is much faster than a regular melee, and when surrounded by a horde and trying to get them away without shooting your friends in the process, making it an effective method. With computers, you can enter scripting to double melee rapidly with little intervels in between, making you able to literally fight off an entire Horde. The use of this exploit is looked down upon in versus mode due to it not only being a glitch, but also by how it gives the Survivors an unfair advantage against Infected players.

Tips

 * Hold the controller so you can still aim and walk while double meleeing, this makes it so you become a basic walking boxer, killing all zombies without wasting ammo.
 * If you are alone and hear the distinctive growl of the Hunter, simply stand still and double-melee repeatedly, when the Hunter pounces, he will automatically fall into the butt of your guns.
 * Do not use in versus mode as you will likely be kicked and called bad names.
 * You may also get kicked in normal online campaigns by other people who want a challenging game. Computer controlled players don't seem to mind, however.
 * Please note that this is against the rules to use in league matches; this is because it is considered a glitch / exploit and you will automatically forfeit the match if the other team reports you for using it.