Or if you know of people who had affairs. How did it come to light?
My dad (he's not dear to me!) had an affair. He was grumpy and horrible to be around for about a year and then my DM fell seriously ill. 2 years after that he left and asked for a divorce.
Turns out he had been having an affair for about 3 years with someone at his dance class. My mum did some sleuthing and they had been caught by her husband but all of them decided not to tell my mum (!) and they just carried on the affair. They now live together but my dad still insists they are "just friends." eh.
My friend and her married boss:
She had always liked him and they got on well. One day she told him how she felt and he said he felt the same. Two days later they made decision to tell their respective partners the truth. They told the truth from the beginning but it didn't mean it was any easier.
Another friend met someone at work and he left his wife for her. They snuck around for two years because her parents were very religious (even though he had already left his wife.)
Two people I work with were having an affair. It was so awkard at the Christmas party when I was sat next to them and someone was shuffling at my foot trying to rub it with theirs. I turned to him and said, "I think you want her foot." She spent most of the evening rubbing his crotch. It was grim. She's now divorced with someone else and he's still with his wife but with no social life.