When crouching on top of a block with a trapdoor on top of it, you can open the trapdoor in order to stand on the block underneath it. If you crouch onto the trapdoor, and then close it again, you as the player will un-crouch when you look at your player model in third person view. But, in first person, you still have the "slightly lower point of view" that you have when sneaking, creating the effect of still sneaking when you are not.

This will happen regardless of what block you are standing inside of, but I noticed it first when I used trapdoors.

Upon exiting the block you are standing in, your character will re-crouch, but the point of view will still be the same. This creates the effect (that I previously created this bug-report for) that you don't recieve the "speed-penalty" for crouching when inside of a block.

You can recreate this by going into a world, place a trapdoor down on the ground, open the trapdoor, stand on top of the block it's on, and then close the trapdoor. 

Example of how it looks: https://youtu.be/ZVViUlPLbfY