RB67 users you'll understand. Last Sunday night doing some still lifes with a friend in her studio, took the last shot on the roll with my RB67 90mm and extension tubes #1, and #2, put it off to the side. Later I'm packing up to leave, I pull the lens off and turn the mirror up switch back to normal and the shutter closes!!! Oops dummy me forgot to stop the exposure, so I lost that shot, it would have been a good one, good thing I took two.