Always Knock Before Entering
One of my friends is a dermatologist, and I asked him to take a look at a birthmark on my lower back. It was at a rainy BBQ and we went to the bathroom for better lighting. I pulled my summer dress up and bent a bit over the sink so that there were no shadows on the region around the birthmark.
At this moment, another friend opened the door. She made a weird noise and closed the door immediately. It took us a few seconds to understand why she reacted so weirdly. Story credit: Reddit / Positive-Vase-Flower