That they look almost exactly the same when you accidently put them on inside out. That is, until you try to put something in your pocket, and can't find it. But because the pants look identical inside out, in all other respects, you think you are losing your mind, and maybe these pants don't really have pockets, maybe you're just crazy. You paw at your hips, examining the waistline closely, pulling the pants out, stretching them, hoping to find the pockets.
Then you see the drawstring on the inside of the pants. Oh. Right.