Dont know if this has been done before ?
I was 19 and had just started seeing this nice upper middle class girl who played in an orchestra. I was knelt on the bed with her arse in the air going hell for leather and slapping her buttocks when my Mum came into my room with some of my shirts she had just ironed. I felt like saying look Mum no hands and waving but I doubt she would have seen the funny side.

My sister walked in on me as i was doing my girlfriend doggy style.She came to tell me dinner was ready.Not something i ever want to happen again.

And my girlfriends mum caught us in the act as my girlfriend was going down on me.But its even worse when you have to face them again in the morning.
However i did hear my girlfriends mum going for it a few days later and i couldnt resist a sarcastic comment the next morning.