OT: Baby/Toddler won't keep diaper on...

We have 4 children, and our youngest daughter was the only one that ever did this. lol I was mortified to walk into the bedroom one day after a nap to have poop smeared all over the walls. She's quite the artist. I can see the humour in it now. lol Needless to say, almost every nap time for maybe 2 months, she took her diaper off. Thankfully, only once more during that two months did she have a "mess". We tried everything from tape, diapers on backwards, jammies on backwards...nothing worked. She just stopped on her own.