Any entity other than the player consistently falls through blocks being pushed by pistons. This also includes immobile entities such as dropped items and derailed mine carts. The only unaffected entity was the spider.

To replicate

1. Use the machine showed below.
2. Place the entity on the slime block
3. Press the button in the back. The entity should fall through the floor.

Sometimes certain entities may not fall, but it will typically fall through the block it was standing on.

Note for 1.7.10, replace the slime blocks with stone, the bug exists in this version as well, and was most likely in other previous versions.