Keys randomly become "stuck" (as if they are being pressed when they're not). Affects keyboard and mouse input.
This is a major bug considering that if this happens while fighting mobs in hardcore, or moving in close proximity to lava or sheer cliffs, the player may easily die.
Indeed, I was just confronted by a creeper and almost died because of this. It isn't the first time that has happened.
This problem has existed since at least 1.3 when singleplayer was changed to use an internal server.