Never? Basically, if you're wearing tight pants/boxers/shorts/whatever, and you do a lot of walking, it rubs you. It usually happens in one place, as you keep walking, and it just gets worse as you walk, and starts to really chafe. It soon starts to really hurt, and it gets worse and worse the more you walk.
It also seems to happen when you're sweaty for some reason, very randomly when you're sweaty. Since you're a PE person, i'm surprised it's never happened to you.
Also, since it hurts so bad, it means you've got to walk bo-legged so it doesn't hurt, which makes you look like a ****...