The Terror of the Toilet
If you ever have the chance to travel to Japan (or if you're lucky enough to live there), you would be wise to avoid the third bathroom stall on the third floor of any school building you visit.
That particular stall is said to be haunted by a ghost with bobbed hair and a red skirt. Known as Toire no Hanako-san (“Hanako of the toilet”), she's the spirit of a girl who met a tragic end at the hands of a demented adult.
If you're in a bit of a spot, and the stall in question is your only option, knock three times to see if Hanako-san answers. If she does, DO NOT OPEN THE DOOR, unless you fancy being pulled into the toilet and joining the ghost girl in her foul-smelling grave.
(The information above comes from Pink Tentacle, which should really publish a travel guide.)