Wall-walking

Wall-walking was a method of utilizing a game's physics in such a way to bypass almost any obstacle.

Controversy
Wall-walking has long been a popular method to reach unfinished content that is not accessible via normal or intended transportation or movement. Players who enjoy exploration often utilize wall-walking to reach areas that are either unfinished or were developed as eye-candy. Blizzard removed a popular form of wall-walking in patch 1.1, although variations of the technique are still usable.

Wall-walking was used to reach closed areas such as the Ironforge Airfield, Shatterspear Vale, and non-instanced versions of Zul'Gurub and Ahn'Qiraj. (Mount Hyjal has a "No Man's Land" debuff that teleports the player out even if the location is reached.) With the arrival of flying in Azeroth in World of Warcraft: Cataclysm, the whole world will become accessible by flying, eliminating the need for exploits.

Patch 2.4
As of patch 2.4, Blizzard made an undocumented change to the game's physics, practically disabling wall-walking completely. Although certain methods may or may not remain to some extent, the ability to wall-walk has been diminished significantly if not completely disabled since the release of patch 2.4. Easier to reach areas that do not require extensive or complex wall-walking to reach may still be accessible.