Talking about statistical probabilities, if there is a 50% chance that any particular flat pack assembly can be done the wrong way, then I guarantee that my own chances of getting it wrong would be 90%. Here, a very simple wooden shelf requires two side pieces to be put on the ends of five shelves. The eagle eyed will see that I put one side on upside down creating an asymmetrical shelf which wouldn't work. I spotted the error after attaching the whole thing. This required unscrewing and re-doing the 30 screws which had been incorrectly attached.