Falling of specific blocks whilst 'crouching'

Whilst crouching, you can still fall off certain blocks, like the enchanting table

Crouching is supposed to stop you from falling, but not on the following blocks:
Enchanting tables, chests, ender chests, daylight detectors, end portal frames (also with E.O.E.), heads and soul sand.

How to make it occour:

1.place any of the above
2. Get on top of it
3. Try to move off it whilst crouching
4. You will fall