Picture the scene, it's just before lunch on a Friday, a Good Friday no less, and you get up from your desk and go to the toilet. (For the remainder of the story to work, you have to imagine you're male.) In you go, walk up to the urinal, undo your flies and start looking (or I guess feeling as you tend to be able to this sort of thing without actually looking) for your, ahem, member. A few seconds go by and you are still looking (feeling), where is the convenient hole in your boxer shorts? Eventually you lean forwards and have a gander at what is going on inside your trousers. The convenient hole is, inconveniently, missing. After solving the how to piss problem, you realise that yes you did in fact put your underwear on back to front this morning.
Now I'm not saying that actually happened to me today, but if by some chance it did... I would have to wonder how drunk/hungover/tired/asleep/stupid (delete as you think applicable) I was when I was getting dressed this morning. I'm pretty sure I've always managed to figure out how to put a pair of boxer shorts on before.