My ex-girlfriend an her family basically had a open door policy when it comes to the bathroom. When someone takes a bath or a shower it was completely normal for them to leave the door open so someone else could brush their teeth. I'm not used to this, so when I stayed there I always locked the door when I wanted to take a shower.
One day, when I was undressing in the bathroom, my girlfriend knocked on the door and asked me to open the door so she could dry her hair and do her makeup. I unlocked the door, she came in and locked the door behind her, she did what she had to do and I took a shower.
When she was finished she just walked out of the bathroom (or more like stormed out because we were in a hurry) leaving the bathroom door completely open, leaving me butt naked in full view of anyone who happened to walked through the hallway.
For a split second I hoped my girlfriends younger sister would walk into the hallway, but that didn't happen.
So I leaned over to grab the towel and at that very moment my mother in law walked out of her bedroom. She stared at me and then at my dick and then yelled: "Aaaahhh, you're naked! Hahahaha!"
She quickly turned around and walked back into the bedroom and I heard her laughing and my father-in-law asking with his deep voice about what is the matter. I heard my mother-in-law laughing and explaining that she completely unexpectedly saw me naked. And I heard my father-in-law mumbling something I could not hear.
After this I quickly closed the door and got dressed. Even to this day (some 12 years later) I wonder why she had this reaction. Her family was used to walking to and from the shower butt naked, and leaving the bathroom door open. My brother-in-law (her son) was seen numerous times by his mother. But I guess she was surprised because I always locked the door before.