One day I decided to go on Snapshot 16w21b, and place an Armor Stand, but when I broke the block below it, the Armor Stand would not fall down. I dug down deeper, and it still didn't move. I tried pushing it down with a piston, but it did nothing. I tried it with all the sides of it, and nothing happened. But when I reloaded the chunks, the Armor Stand fell down perfectly. I was in NOT in Multiplayer. I also tried this in Minecraft 1.8.7, and it happened again. The next time I tried, which was about a day later, the Armor Stand fell down perfectly, without problems.