Occasionally when you press w (for forward) you will continue moving forward even after you have let go of the key. Because this is so rare it may be a combination of keys pressed at a specific time and /or order. I'm not sure how to duplicate this bug but there is proof in this video at 8:48 http://www.youtube.com/watch?v=ygHY6DnVb7M&feature=youtu.be&annotation_id=annotation_3828244553&feature=iv&src_vid=lRaYJb97mOQ
This bug can also happen in survival and creative (even when flying) and in both Singleplayer and Multiplayer.