Walked in on my clothed father sitting on the bathtub edge, soaping my mother's bare breasts as she bathed. Oddly she wasn't startled, in fact had this dreamy, detached look with a vague smile, and my Dad very calmly said, in his normal voice: "Your Father is helping your Mother to wash. Close the door, Robert." I think it was during a period of time when they had disabled that bathroom door's lock, to prevent me from barricading myself in there whenever I got into trouble.
I was something around 10, and kinda surprised because I didn't know my rather formal & reserved parents did that kind of stuff. We rarely think that about our parents, do we, especially when we're kids?
But the tables later got turned on me, with Mom barging into my bedroom a few times in my early teens while I was beating off. She closed that door so fast I had to blink and wonder if I had imagined it. And it was never mentioned.