Minecarts are extremely buggy in 1.6.1

What I expected to happen was that I would appear riding normally in a minecart.

What actually happened was it appeared as if the minecart was floating above me and I was sunken into the tracks, and sometimes the other way around.

Steps to Reproduce:
1. Get in a minecart
2. Try going up and down hills.

I have attached some files to show what it looks like.
I was in survival at the time but it probably does the same thing in creative.