Using default keybinds and holding shift+space+w will not make you go forwards, you will be stuck in place. Some more bizarre effects come from hitting another direction directly before "w", leading to going that direction instead. 

EDIT: Another glitch with Crouch-Flying is that if you are sprinting in the air, then start Crouch-Flying, you will start falling in Y position, rather than having a constant Y.

EDIT 2: Turns out that this was just an issue with my keyboard driver. This issue only happens with that key combination, so it was unnoticed until I played Minecraft, so I though it was an issue with the latest snapshot. – this is NOT a Minecraft bug, PLEASE IGNORE.