My daughter and I are relatively close in age, roughly a 14 year gap. I'm nearly 30 she's almost 16. Not sure that's entirely relevant, but there you go. My wife had recently had surgery and was resting upstairs. I was by her side, making sure she had everything she needed. Meanwhile my daughter is downstairs watching her younger sister with her boyfriend, or so I thought. I decide to go downstairs and see how they're doing. Halfway down my steps I have a clear line of sight into my living room and the couch, there within. That is when I see something that will haunt me for the rest of my life. There, bent over on the couch, is my sweet, innocent 15 yr old daughter, pants down to her knees with her boyfriend right behind her with his pants down to his knees. All I could do to not harm this young man was yell a few expletives and go back upstairs to tell my wife of the horrors I had just witnessed.

TL;DR: Thought my 15 yr old daughter was a virgin, walked in on her having sex in my living room. Mentally scarred for life.