When you go on your back while prone, and start moving whether you are pressing any movement key
(W, A, S, D)
and you press the C key to crouch or stand up, it won't let you get up, and instead, just raise your camera to the crouch height or standing height.
This can be especially frustrating when waking up from Unconsciousness, laying on your back by default, and moving away, whilst trying to stand back up (moving away from zombie) and you die from it :(