The mud puddle that is in the doorway to his stall.
Seriously. I watched this pony jump that thing several times. Mostly going out of his stall while I was getting his grain. But he also jump into his stall a couple of times. They were nice little jumps (not hops) and he managed to stop before hitting the door on the opposite side.
He ceases to amuse me. And for that, he is worth his weight in gold.