This happened about 20 years ago. My in-laws were, and still are, divorced. I was at my mother-in-law's house with my wife at the time to spend the day. While we were there, by wife's friend from high school drove by and saw that we were there, so she stopped by to hang out. They decided to go to town to another friend's house for a little get together. My wife asked me if it was OK, and I said yes, no big deal, I'd just stay there and watch TV, or whatever. While this was happening, my MIL was out for a walk and came back in the back door and slipped downstairs to clean up. During this time, I walked down the hall to the bathroom, and when I came out, there stood my MIL, naked as the day she was born. We both froze and then she got this little smirk on her face and I took back to the bathroom where I spent about 20 minutes. We never talked about this, but I do know that she didn't know we were there yet, since my wife took our car in to town. I always wondered about that little smirk. Even before this had happened, I always, kind of deep down wished my wife was built a little more like her mother, instead of having the more sturdy build like her father. It was always really awkward between me and my MIL anyway, and it was awkward for the next 8 years or so, until she got hit by that bus. I always thought she looked at me funny, but never had the guts to ask her if she was weird or what the deal was.
My father-in-law and I, however, get along great and are really good friends!