Back in the good ol' days, our guild was much smaller and tight-knit. We had enough for a solid 25-man raid, with a handful others who just liked to call us "home." Some guildies knew each other in real life or were roommates who liked to have fun and play pranks on each other. While I glad to not be a part of it, I did enjoy hearing the creativity they came up with, especially when it involved something in game.
The most memorable prank was back in the golden days of yore, back when mounts were items in your bag, guns and bows took ammunition, and gold wasn't fished in fountains. Someone logged onto a low-level alt and commented the character wasn't where he had left him. He shortly discovered that he had no gear, no gold for a flight path, no hearthstone, no mount, was nowhere near a bank, and had to run through high-level mobs to get anywhere remotely close to bank access. Watching him discover all these atrocities one at a time made for a very entertaining evening.
With authenticators bringing heightened account security and Blizzard making items such as mounts "learned," pranks such as this are hard to recreate. This just means pranksters have to be more creative.
Do you and your friends pull fast ones on each other? What epic in-game tales and pranks were you in on or have you heard about?