Our house mate walked in on us once.
What are your experiences of being interrupted or interrupting someone else...
The guys I live with usually find an excuse to walk in on my girlfriend and me when we're having sex, usually naked after they get out of the shower, looking for underwear! (Likely excuse!)
David and I stumbled upon a young couple having sex on the cliff out at Blackhand Gorge where we usually set up to do a photoshoot. They scrambled to get dressed and leave. Too bad. They were pretty hot! :-)
Parents and siblings, but none never figured out what was going on. Thankfully.