There was a real classic a few years back. A group of students got talking to an Irish navvy about to dig up the road in preparation to lay new cabling down. They then went round to the police station and told the receptionist that a man was about to dig up the road as an April fool's stunt, walk off and leave everything undone! To make sure more chaos followed they then went back to this thicko and told him that a student masquerading as a policeman was coming round to arrest him to which he laughed his head off and said: "Thanks, I'm ready with a bucket of cold water." Needless to say neither were very amused when they realised they'd both been had!
The big Daddy of them all though was when the BBC told viewers that they'd found out who didn't have a television licence and this is what would happen if they didn't sort thing out within one calendar month: they then merely unplugged the microphones and carried on talking for the next two minutes with, of course, no sound. It backfired spectacularly! Not only did viewers buy it, the next day the Post Office was swamped out with television licence application forms! 
