The cast of "ER" is known for its on-set pranks. George Clooney was a real devil at this, but others soon got in on the act.
Here's a brief explanation of once such gotcha ...
There's a great video here, too, about the pranks -- booby traps, clowning around, greasy balls and more.
You can see Clooney's pranks galore in the gag reel posted here, too.