Nitpick: As an avid Tetris player, I have to say that the examples given for "extremely rare" events actually happen quite often during normal play. In particular, clearing 4 lines at once, two times in row will generally happen within the first few minutes of game play and is sort of the whole goal (if you're trying to maximize points). Wall kicks are less common but certainly not rare enough that their processing code would be left uncovered after playing a few games.