A bug occurs in the game when a minecart of any type is shoved into a block by a piston.

When the direction the piston is pushing is North or West it "clips" though it. It shows the minecart being pushed into the block and it is temporarily stuck there and it turns black but when the piston retracts it teleports back to its original position, but when the direction the piston is being pushed in is south or east it stays there and is not affected by the pistons action.

To reproduce this bug place a sticky piston facing north or west and put a block in front of it. then get a minecart that is not on rails right in front of the block that will be pushed by the piston and make sure there is a block for the minecart to be pushed into then activate the piston with a button.

Here's a video of the bug in case you didn't get that.

http://youtu.be/D2M7DBOxwys