While playing in survival, I placed a 3x4 wall of Cobblestone Wall blocks. When breaking them, I got a lag spike and 1 of the broken wall blocks disappeared.
It seemed to be a "memory leak" as it raised the memory usage of java a good amount, caused the world to be laggy. Had to close and re-open to level it off.

I happen to be recording at the time for my weekly series. Here is the link to right before it happens:
http://youtu.be/zEIjvxuqVM8?t=13m18s (Happens at 13:21) and then starting at 13:40, you can see it lagging.