Well, those sort of pranks are only fun for the pranksters. I've always had three rules when it comes to practical jokes. No Permanent Damage. While this is a good rule to live your life by, it's doubly true of pranks. Now, permanent damage can occur in many ways.
Physical injury is the most obvious.
Don't do pranks that hurt people or that might lead to being hurt. This seems like common sense, but it's easy to get caught up in a cool idea and decide to climb onto a roof or pour a bucket of goat blood off an overpass or point your pellet gun at a cop. However, other types of damage are equally important. Psychological damage counts, and in those cases where you're not sure if you might cause permanent damage it's always important to err on the side of caution. What you consider a funny prank might prove to be someone else's terrifying psychotic episode.