Placing a slab, a trapdoor, and then a block on top will have weird behavior when shifting inside. 

when you're inside you can close the trapdoor (normally without the slab, you would go into "swimming" mode) then you are able to unshift even when the block is over your head.

Instruction for reproducing.

1. place a slab with a trapdoor on top of it

2. open the trapdoor

3. any block on top of the trapdoor

4. shift then get on top of the slab and under the block

5. close the trapdoor then stop holding shift.

Expected: You would get pushed out still be shifting or go into "swimming" mode.

Results: Your character stops crouching and you go normal speed.

I haven't looked too far into it, but I think it's a problem with the script or code detecting if you can stop crouching. But your point of view doesn't change, it stays at the crouching postion.