An Iraq war veteran came home in April 2007 and suspect his wife of cheating on him with another guy. A few months later, he ran through the collection of Miis on the Wii and found an odd — and familiar — face. It turns out wifey and the other guy played Mii bowling on multiple occasions and by tracking play time on the Wii calendar was able to see when they were playing together. Clever, no?
I suspect this is an urban legend/hoax, but just remember: Grow the freak up already and stop playing Wii with your man-hos and mistresses.