This happened a few years back. My wife and I were living in VT at the time, and would travel down to NJ to visit her family about once a month. My wife is Puerto Rican, and her and her family are all about LA FAMILIA... So I've become accustomed to not having any privacy anymore when around them. They often all hang out together in her Mom's bed for hours, just talking - a completely foreign concept to me.

So I was out seeing some friends while my wife stayed home visiting with her Mom and sisters. I got home late, around 1 in the morning, and went to her old bedroom upstairs where we generally stayed when in town. Sometimes her Mom would give us her bedroom, as the bed was much more comfy and large, and we would be exhausted by the 5 hour drive. I proceeded to get down to my birthday suit, as that's the only way I can sleep now (I'm from the West Coast and a desert climate - this East Coast humidity kills me and my little friends). I climbed over her, as she took the outside of the bed, all the while unconsciously rubbing my junk all over her. I got under the sheets, got in my usual large spoon position, and laid my head on the pillow... It was her hair, of ALL things, that I noticed was first. It took about 2 or 3 seconds of lying there before I then ran my hand down her side and it just didn't have the same (youthful?) feel.

I instantly shot up out of bed, scrambling for my belongings on the floor - all the while naked - and ran to the master bedroom to find my wife (double checked for accuracy this time) sleeping. Told everyone what happened the next day, as I didn't want my MIL to be secretly awake or something, and wondering wtf had gone on last night. We all laugh about it today, afterall, we are Familia.

TL;DR - Wife and MIL switched bedrooms, didn't tell me. I got naked and in bed with the wrong one.