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.