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