I have lived in several different houses with a bunch of guys. Needless to say things got pretty rowdy sometimes and many were victims of some pretty funny jokes. One of the favorites as I recall (and still is) is to go into the bathroom while the victim is taking a shower, and pour a bucket of extreeeemmmlly cold water on them over the top of the shower curtain. This is quite a shocking experience, and if you are fast enough you can get away before the victim finds out you did it.
I remember one guy I lived with getting this all the time. One time he got sick of putting up with it and jumped out of the shower into the hall squirting shampoo at everyone in sight. The next time this happened the guys were ready with a camera to take pictures of him as he ran out of the bathroom. These pictures were later shown at his bachelor party.