When you mean to do something really obvious but then you forget what you were doing and end up doing something something completely
different. i.e, when you have a wrapper in one
hand and food in the other, and throw away the food and
try to eat the wrapper.
Brain Fart:
Person 1: Man, I had such a
brain fart
today.
Person 2: What did you do?
Person 1: I was holding food and a wrapper and threw away the food and tried to eat the wrapper.