I've had 2 who've done this. The first one fell over the side and it scared her so much she never did do it again. The 2nd one would try from time to time but I had him on a video baby monitor so I'd radio down and tell him to lay down. More often, he would just stand up and scream/cry and call out to the others (he slept in a different room! Lol) so I started putting a crib sheet over the top of his playpen. It usually worked!!