When a Minecart, if you throw a snowball at the cart, it will destroy it, and you will sink shoulder-deep in the ground. You have to jump up to get un-stuck. This occurs while the cart is on rails, and you are in creative mode. I have repeated this, and it works every time. I have attached step by step pictures.
Environment
WIndows 7, Java - Version 7 (Update 25), Vanilla Minecraft, 64 Bit OS and Graphics.
Applies also to destroying Minecarts with arrows