Be on guard day and night, but best friends are hard to guard against

Have you ever been seduced by your best friend? Or by your girlfriend/wife's best friend?

I was playing mahjong with a friend recently, and during the chat she said that one of her sisters found her best friend's sexy underwear in the back seat of her husband's car, and then she hid a recorder in the car, and found it a week later, it turned out that they were together...