My previous user error was always forgetting to close the charge port cover - unplug, get in, realize, swear, get out, close, get in... I managed to break that habit.
Now it is the parcel shelf. Whenever I put groceries or anything in the trunk I fold up the parcel shelf for better access, and almost always forget to put it down again. I close the trunk lid, drive away and realize the first time I look out of the rear view mirror that I can't see a damn thing. Doh!
Don't they say the biggest risk in a car is the loose nut attached to the wheel?