On April Fool's Day of 1957, the BBC broadcast this video, which has been called the greatest hoax ever perpetrated on TV viewers. I don't know if whoever said that considered some of the shows I worked on which were advertised as comedies…but what the BBC did was truly impressive, given how many viewers apparently didn't get that it was a prank.
I wrote here the other day that I think most pranks are infantile and an awful lot are just nasty. The exceptions would be those that don't hurt or embarrass anyone and which have a bit of wit about them. This one would qualify…