Basically you can jump over stone walls. I noticed it on a server I was playing on and reproduced it on single player in creative and survival modes.

What I expected to happen was...:
I expected to not be able to jump over this wall into a pen

What actually happened was...:
I was able to jump the wall quite easily

Steps to Reproduce:
1. Place down a cobble stone wall at least 2 meters in length
2. Walk towards the wall and jump
3. You will jump on to or over the wall as if it had a 1 meter hit box.