I Just noticed on mojangs website:

Here’s the pre-release for 1.6.2! We’re planning to post this release on Monday (July 8).
**FIXED HORSES STARTING TO SUFFOCATE WHEN TOUCHING WALLS**

-----------------ORIGINAL-BUG-DESCRIPTION------------------

If you ride underneath a tree, your head goes through the leaves, but whenever you ride underneath a proper block, (Stone etc.) and then jump, you will start suffocating in the block. (I think this could be fixed by just hitting your head as normal when you jump underneath a block.)