The Wall Climbing Glitch (known as the wall hop in the obby community) 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 but unfixed at an unknown date after 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, jump holding W and flicking their mouse either right or left. You used to be able to use the < and > keys to climb walls, but you can do it on R15, but it's much more difficult. On mobile, if it has shiftlock, jump at the right time, then flick.
This glitch can also be done using the < and > keys, as they turn your character 45 degrees in either direction. This only works with some kinds of mechanical keyboards.
To practice, build a tall wall of terrain and attempt this trick.
At an unknown date after the fix, the glitch has been unfixed, meaning it can be performed once again. But, you will rarely be bounced away from the wall you were attempting to climb due to ROBLOX's physics.