There was this time when a girl was trying on new clothes in her bedroom. She was in the middle of changing and was completely naked when her mother suddenly entered without knocking. The girl felt so exposed and violated in a sense, even though it was an accident. Her mother apologized profusely and they had a long talk about privacy. Since then, they have made a point to always knock on each other's doors.
Well, I heard of a situation where a young boy was changing clothes in his room and thought the door was locked. But his little sister managed to open it and saw him naked. He was so shocked and embarrassed that he hid under the bed covers immediately. It was a really awkward moment for both of them. His sister was just as surprised as he was, and she quickly ran away. After that, it took a while for them to act normal around each other again.