You have to do your makeup somewhere other than usual and you have forgotten to bring something important like the eyeshadow base or the mascara?
I slept at my friend's place yesterday and had brought my makeup bag since I wanted to do my makeup in the morning. I did all my makeup before realizing I had forgotten my mascara.... Smokey eyes doesn't look all that amazing without mascara, haha :P
I've also left the house with a full face of makeup but without my eyebrows on once because I was in such a hurry... Now, that might work if your brows are the same colour as your hair but having blonde eyebrows and black hair doesn't really match, lol :P
Have you made any stupid makeup-forgetting mistakes like that? ;)