When riding a pig which is riding a minecart, your body spins and your head doesn't.

The ability to ride a mob in a minecart was added in 13w09a, but this is a bug that comes with it:

The pig spins around when in a circular track, but not in a straight one.
The body aligns with the pig, but the head doesn't.

Here's a video of it: http://youtu.be/y7jGL4_Pcyw