Probably the best "joke" we played was the night we changed format on KAAY's FM sister station, KLPQ (at the time). A good joke, certainly an attention getter, and not on April 1.
We began playing "Baby Elephant Walk" and "We will Rock You" back to back, and over and over again. The reason for playing Baby Elephant Walk was because that was what was played over and over again as the announcers read the Little Rock phone book on the air, when KTHS changed to KAAY.
Needless to say, we did get some phone calls asking us what the $&@# was going on. One listener actually called the Little Rock police to "go to the radio station and see what was wrong". When the police officer arrived, he was invited in, shown the studios, and clued-in as to what was going on.
KLPQ eventually became KKPT in Little Rock, and is now a Classic Rock format station.