Since 1.4.6 I've had this happen regularly, no matter what game mode I'm on, whether on a server or in single player. Every once in a while, when moving and placing a block, whatever keys I'm pressing will "toggle" on. So, if I'm moving forwards and building, it will cause me to keep moving forwards, even if I don't touch the keyboard. Many times, this has resulted in me falling to my death. Just opening my inventory fixes it, but with a slight delay.