Java 7 Update 7
Windows 7 Home Premium
Intel Core i5
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