Quite possibly the all time greatest on air joke was when KTHS changed formats and call letters to KAAY. The "Baby Elephant Walk" was played back to back and over and over again, non stop, while the Little Rock telephone book was read on the air. It certainly drew a lot of attention to the station, and was the talk of the town for many following days. (Not on April 1, though).
A recording of part of this can be found elsewhere on this blog -
(I believe this is what Dave is mentioning...check this link bs)
(Although this actually happened on Labor Day weekend in 1962, this WAS a pretty good prank! The saddest joke was when KAAY "went to be with God" on April 3, 1985 bs)