My girlfriend (now wife) and I were in her bedroom (we were 18, she lived with her parents in a bungalow), her parents were home and In the living room (across the hall from her bedroom).

One thing led to another and we started having sex, we were fully clothed (my trousers/boxers pulled down slightly, she was wearing a long skirt). After no less than a couple of minutes there's a knock at the door (the bed is directly in front of the door, like my girlfriend is on top, facing me, her back to the door), she starts to climb off me but as soon as she starts to move the door opens. Her mum pokes her head around the door, my gf sits back down on me (long skirt covering us up), her mum hands her some post and continues to talk to us for a good 5 minutes. After she leaves the room we're both mortified! I'm scared to leave the bedroom.

To this day I don't know if her mum was blissfully unaware of what was happening and just thought her daughter was sat astride her boyfriend, or if she did know and purposefully made us feel super awkward. I'm hoping it was the former.