This is basically the opposite of mc's thread. Post any card fails you have either done or witnessed. They can be funny or embarassing.
For instance, playing Allure of Darkness first without thinking and discarding 6 cards X_x
Or setting Pot of Greed face-down (yes my friend did this one time when he was still learning the game. It was even better because I MSTed it)
One time I saw a kid at a tournament trade a real card for a created card
What silly things have you guys experienced?