The Wall Climbing Glitch used to allow the player to vertically scale walls and go to places otherwise inaccessible and has been performed from early 2013 to 2015. It has been fixed since early 2015.
To perform this glitch, the wall the player intends to climb must be made of separate parts. Players can identify separate parts by finding parts with different colors, different materials, or outlines. This glitch always works on walls of Terrain, as each 'block' of terrain behaves like its own part.
Next, the player has to press up against the wall, then wiggle around and hold W.
The last step is to press two keys. One of them is the W key, and the other is the key in the direction you turned. If you turned left in the above step, hold down A. If you turned right, hold down D. After this is done, press and hold space (while still holding down W and A/D) and you should propel yourself up the wall.
To practice, build a tall wall of terrain and attempt this trick. Although there's no point since it was patched in 2015.
A game called Super Fun Obby has a stage where you have to use the wall climbing glitch in order to progress to the next stage. It's now impossible.