I walked in and saw my brother-in-law fully naked one day. We were traveling and had stayed a few nights at my husband's brother's house up in Central Pennsylvania. His brother was up early every morning and left for work long before my husband and I would get up. One morning, I heard the hall shower come on, and I was feeling aroused, so I decided to surprise my husband. I went inside the bathroom and dropped my panties on the floor on top of his boxers. I remember the thought hit my mind that my husband wears boxer briefs, but that was as far as the thought went. (I should have realized those were not my husband's underwear.) I slid into the back of the shower inside the curtain, and my brother-in-law turned as our eyes met and locked. Our bodies just froze. We were standing 12" apart, both totally butt-ass naked.
I apologized profusely, but he never acted upset over it, and it was years ago, and we still stay with him when we travel to visit. I fondly remember my husband's brother is much larger endowed than my husband. There is a significant difference between the two of them.
Apparently, that morning, his brother was going to work late, and my husband was downstairs already drinking coffee when I got up. I now knock before going inside any door in their house.