Since my earlier years were spent watching my cousins play video games as much as they were actually playing them, I can't really think of any real "noobish" move I've made, as I've always had a generally good grasp of how to play the game.
However, there are some stupid things I've done, usually involving tactical RPGs and making a mistake with a move or an action. In those games, even one screw-up like that can be deadly.