There are more ways to ruin a photograph than you would think. All these potentially fatal mistakes are specific to large format, in the sense that most of them are impossible, or quite rare with 35mm and MF cameras. Here are some of them, and how to avoid making the mistake.
Additional suggestions welcome.
To avoid fatal confusions Be sure to operate in a systematic way, to always do the actions in the same sequence, and to double check everything. After I am done focussing and composing, the first thing I do is close then lens. Then I meter, and set the f-stop and shutter speed. Then I insert an holder, making sure it is white. By checking that I put the black side of the dark slide back, before removing the holder, I make sure that the exposure was done.