Later on, I joined a different guild with different pranks -- most of them revolved around the fact that a good chunk of the guild was a bunch of guys that lived together or near each other. These guys all had each other's account information -- yes, it was a bad idea. No, you should never, ever share your account information with anyone; it's not only a security issue, it's against the ToS. However, there is also another reason you shouldn't share account information that was beautifully illustrated by the guild. See, we often grouped together to raid Zul'Gurub, AQ20 or AQ40 -- and if we were short a tank, one of the guys would log on his buddy the tank's account and tank the raid for us. That's pretty much why they were sharing accounts, so that we would have fill-in options if someone couldn't show for a raid.
In the unfortunate instance that someone was feeling squirrelly, however, the situation could quickly turn for the worse -- or amusing, depending on which side of the login screen you were on. A player who was out of town for a week on a business trip would log back in to find his character lovingly planted at the bottom of the ocean, naked, bags emptied (into the bank, of course), with his hearthstone deleted. There were plenty of popular locations to strand people -- a little traveling could provide some truly entertaining locales in which said players could be stranded.
Nowadays in my current guild, we have the pre-raid doldrums, that half-hour or so while the raid is being put together and nothing is going on. Since we're all sitting in Icecrown, we can see general chat for the region -- and post in it. The favorite thing to do at the moment is to pipe up in general chat with a "LFM for Chillmanders, PST to <guild member name>," including the name of someone in Vent at the time -- bonus points if he's not actually in Icecrown, so he can't see you do it. Then wait to see how long it takes the named guildmate to start swearing in Vent because he's been riddled with tells out of nowhere. Retaliation is usually swift.
Nobody in any guild I've been in does this to be mean -- more often than not, it's a matter of lovingly harassing people or doing it for the laughs. Do your guildmates play pranks on each other? Does your guild include a fleet of mages that enjoy playing portal roulette? Do you have that "one guy" in the guild who incessantly plays practical jokes on people? Are you that guy? If you're the victim of a prank, do you get upset, or do you just laugh it off?