Minecraft 14w20b, Minecraft 14w34d, Minecraft 15w34a, Minecraft 15w34b, Minecraft 15w34c, Minecraft 15w34d
Windows 7; 64-bit; RAM 8GB; 1.90GHz Processor; Java 7
When player entities are standing on blocks moved by pistons, they may randomly fall through the floor when the piston is pushing blocks. It is more consistent when the player is pushed by a block from a piston as well.
1. Use the machine shown in the pictures below
2. stand on the slime block, and press the button in the back
3. repeat and eventually the player should clip through the floor.
I apologize for the length of my previous report. I hope this helps in isolating this particular bug.