Invented by the great Steve Caballero (Caballero, caballerial, get it?), the caballerial is a difficult skateboarding trick that is fun to see done, and even more fun to hurt yourself trying to do.
The caballerial was invented on vert, but has been done on flat many times since. What it consists of is a fakie backside 360 ollie. In laymen's terms, you are riding backwards on your skateboard, ollie, and rotate one full rotation while in the air, thus still traveling backwards when you land. See? Sounds pretty difficult. If you do it on vert though, you land traveling forwards again, which makes it somewhat easier.
The caballerial also has spawned a somewhat easier trick, the half-cab. It's quite a bit easier, especially if you are doing it on flat.