Armour stand disappears, reappears on game restart.

OS: MS Vista, Java: 1.8.0_25

I had just crafted and placed 2 Armour stands, one without any armour placed on it and the other had leggings.
I carried on playing the game, focusing on building. I came back to my armour stands after around 30 minutes and found that they had both disappeared along with my leggings. I was no more than around 200 blocks away from them, I hadn't died/respawned throughout this period.

I restarted the game to try to resolve the problem, they did reappear (along with my armour), but within the stone slab they were originally placed. Had to remove the slab to get to them.