I was testing the slimeblocks in the last snapshot, I wanted to bounce my minecart up, but it crashed my minecraft.

How to trigger this bug:
1. Place a upwards facing piston with a slimeblock on top
2. Place some redstone and a lever to trigger this piston
3. Place a rail powered rail with a minecart on the slimeblock
4. Mount the minecart and pull the lever
5. Crash