Put the summary of the bug you're having here

What I expected to happen was...:
To be able to jump when inputting the 'W' key and being up against a single block. (I have auto jump disabled) 

What actually happened was...:
As I tried to jump, I would not be able to. However, whenever I let go of the 'W' key I am able to jump when up against a block. Stairs and slabs are not affected by this slight issue. This bug also seems to affect mobs, so far I've only seen it affect cows and pigs.

Steps to Reproduce:
1. Turn off Auto jump
2. Go into a world and walk start into a single block
3. Try to press the jump key while still holding the 'W'
4. You as the player should not be able to jump