Ironaxe

Still happens with 1.8.4. I just encountered this bug today after driving my boat to a dock and getting out of it. The dock consists of a wood slab in between 2 wood planks. The slab is at water level to allow the boat to sit on top and get out of the water. I was able to dock the boat on top of the slab, but I died as soon as I left the boat.

Restarting the client on 1.8.3 worked for me while while playing on a private Vanilla server.

I can confirm this happened to me as well playing minecraft server 1.8.1. I left a minecart sitting on an unpowered rail that is part of a launcher system. It consists of an unpowered rail, normal rail, and then 3 powered rails. The unpowered rail is connected to redstone and a button. Pressing the button launches the minecart. I logged out of the game and then upon return , the minecart was missing. The map is set to peaceful so there is no chance that a mob destroyed it. Given that a minecart costs 5 iron ingots, it is sadly missed.