Imagine a block with a fence post on top of it. Next to that block, it drops off. (i.e. It's a ledge.) Now imagine sneaking/crouching on the edge of that block, next to the fence post. Suddenly, your motion becomes jerky. It's like after you move a little bit, you suddenly teleport back to where you started, so if you just keep the key pressed down it's like you're going back and forth. You may even suddenly fall off the block.

This has happened for a long time, but I only just now thought to post about it.