Usually, when you have your mouse on a block, say, grass, it'll be outlined with a black line. It'll be that way in creative and survival.
Instead, why'll I was playing 1.8 pre3, the hitbox on the grass (or any block) disappeared once I switched my gamemode to survival.
The way you can trigger the bug is to
1. be on creative.
2. put your mouse on a block of any sort.
3. then switch to survival.
The hitbox on the block should disappear.