My great blunders of recent memory happened when I was really tired. It was after midnight, and the game was dragging on. The PCs were tracking a primitive druid across a prehistoric valley. Finally they come across the guy and his dinosaur companion. I put some minis out to represent their foes. Realizing I'm not using the T-Rex mini, they ask if the dinosuar is a velicoraptor. I saw no it is different, and then proceed to call for initiatve. We fight the battle. The adventure ends. Next day I realize my mistake: "No it is different" is not a description of a dinosuar! Argg!!! I should have lost my DM license that day.