I've done worse. I've never told anyone this, but once while out in the field, I opened up the dark slide to see if I had already exposed that side of a film holder. Somehow my brain thought this would be okay, like I could see a fully developed piece of film in there and only the light from inside a camera would expose it. I didn't even think to check the orientation of the tab at the top. I realized what I did half way through pulling the slide and just sat there frozen, trying to figure out what happened. I get caught up in depth of field, bellows extensions, and zone metering calculations feeling all smart with numbers, and then go and do the dumbest thing imaginable.