At 25, my closest best friend - more like a brother, longterm JO bud, would eventually be best man at my wedding - was talking with me in our usual level of dirty detail about fucking his girlfriend. All her positions, noises, holes, everything, it was normal discussion material for us. The conversation shifted and I noticed he was using new words, that a past experience was now replaced by a hypothetical, there was a conspicuously high number of "maybes," I remember chuckling thinking it was like a rhyme. I was thinking in that weird way because we had just smoked up a ton and were stoned out of our minds.
I later figured out he had been suggesting I could fuck his gf with my much bigger cock while he watched. In the moment I was so stoned I didn't even respond and the opportunity passed. I will never stop regretting that one. He's shown me her nudes and I'd have loved to fuck her, but it would have been even better to see him staring jealously at my big dick again like he'd used to.