After pressing X to couch and holding it for about ~2 seconds, it will be stuck and you will be unable to cancel it unless you spam X several times. You can hear me mashing the button very loudly in the video.
In 1.4.1 this bug still exists.
How to reproduce: When couching have no keys pressed. Press w, a or d. Now when you press x the icon on the bottom right shows that you are not couching anymore, however your character is still couching. This also makes it possible to couch at really low speed.
Hopefully this will help you.