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. Prior to the Cataclysm, Mount Hyjal had a "No Man's Land" debuff that teleported the player out even if the location was reached. With the arrival of flying in Azeroth in World of Warcraft: Cataclysm, the whole world became 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.