A total lack of recognition.
A silly mistake.
Can be stress related.
"I had a total brain-fart when she asked for my number."
An unwarranted interjection in an otherwise controlled environment.
Teacher: So you see, we now have only three weeks before the biology exams.
Student: That means I will only have three more driving lessons before my driving test.
Teacher: Uhh.
Student: Sorry sir, just a bain-fart.
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.
When you are attempting to remember something very obvious, someting that you know you should know. This feeling often leads to head banging and hair pulling.
bobby raised his hand to answer the teacher's question only to find that he had had a brain fart, and he was unable to answer 2+2.
A sudden and involuntary (although depending on the source, not altogether unexpected) release of ignorance.
Jeff had a brain fart and locked his keys in his car (again!)
When something you were going to say but the thought escaped your memory and doesn't return.
I have an idea but now I don't remember what it was, it was just on the tip of my tongue--ugg brain fart!