For the first 5 days I had Flash, I kept him in a cardboard box, because I didn't have a cage yet. It was the box my last computer came in, so it was about 2 feet square, which isn't bad for a temp cage. And I thought he wouldn't be able to jump out the top, because I stood the flaps straight up, making it about 3 feet up, which is pretty high for such a small bunny (he's maybe 4-5 pounds). He tried jumping despite the height, and when he hit the flap that was standing up, it just folded outwards, and he came down on the outside, without actually having to jump that high.
When I realized what he'd done, I put him back in the box, only this time, I clipped the flaps together, so they'd stay standing straight up even if he hit them. That didn't stop him from trying. He knew it worked once, so he decided to try it another 3 or 4 times to make his way out again. I kept hearing him sliding against the cardboard and landing IN the water bowl from the other room.