Item frames and paintings pop through walls when hit. Especially irritating when they "teleport" through 20 meters of rock to the top/back side of the mountain you've carved your evil supervillan lai... I mean your humble mine into.

To reproduce:
1- Build lair in side of mountain.
2- Add some class and culture through art.
3- Repeatedly place/remove to find just the right artwork to accentuate your architectural tastes.
4- Curse when the painting disappears, and go on an epic quest to find it before it despawns.
5- Optional - get killed by creepers while hunting for lost painting.