Brainfault is the moment you reach for something and it is not there. The name borrows from computers, where a page fault is when the data you need is not in fast memory and has to be fetched from somewhere slower. In a head it is the blank where a thought just was.
You know there was something you were about to do. You can feel the shape of it. And it is gone, faulted out the instant an interruption took the slot it was holding. Sometimes it returns when you retrace the trigger; sometimes it is just lost.
It is not a memory flaw. It is the cost of holding more live threads than you have room for: see brainwidth. The less free width you have, the more brainfaults you take, and the more often you stand in a room wondering what you came in for.