I backed my truck into the 5th wheel before I had raised it high enough. Dented the top edge of the tailgate after it hit the kingpin.
At least, I've never dropped the damn thing.
Note: don't fuck with anyone while they're hooking/unhooking.
Side note: I have backed, in one shot, the 5th on the side of the house( 2' each side) in one shot while the kids were taking bets on it.