Forgetting you've done something because you do it over and over again routinely. This happens because your brain tends to filter things out after repetitive action. This is why you don't remember every time you blink or find random little chores mysteriously done.
Dude: Wait a minute bro, did I just feed my dog or not?
Bro: Dude, you must have Monotonesia because you just did ten minutes ago.
by Dr Face Doctor August 12, 2010