@Eliusa,
Well, one is caused by the wife insisting on giving the husband a home haircut, causing numerous rough spots, particularly on the tufty bit of the crown.
The other is the supposed G Spot, which most husbands search for years to find, but after decades and with the aid of various torches and magnifying glasses, usually give up, call it a day and divert their energies to restoring old cars.