When I logged in to my world in the latest snapshot 14w25a I encountered this strange bug:
The contents of some (not all) of my chests are lying on the floor in front of them whilst at the same time being in the chests. Additionally there are several dozen stacks of item frames on the ground. I can pick up all of these items so basically this duplicated huge amounts of items.
Because of the 5000 entities lying on the ground the game was obviously lagging and closed itself after about 30 seconds without generating an error message in my minecraft folder. I have included a screenshot of the items on the ground,