Today when i was playing on my private server
I was sprinting around and i accidently pressed the "Alt" key on my keyboard and it completely paused minecraft and my cursor showed up.
I pressed the Alt key again and my game resumed BUT... I was still sprinting and not even touching my keyboard!
I double pressed "space" to fly and i was flying at sprinting speed instead of walking speed.
The only way I could stop it was pressing the "W" key again.