this is up there - it's the masseuse/steak one
Most of the best ones start off as apparently something completely different, where the bizarre/outrageous thing is brushed over nonchalantly as something mundane and then a much less pertinent factor is seized on as the point in debate.
Like the steak one, where OP's main objection was to having to pay for a steak as a vegetarian, when the elephant in the room was her DH expecting to sit down with the masseuse for the meal!
Snapped & Farted was another. Nobody remembers the actual reason for the thread - iirc, it was a petty little tiff - but the crucial plot twist was clearly in the by-the-by-mentioned snapping and the farting, especially the farting!
It's the MN equivalent of when I was on a train, travelling along the North Wales coast, when a group of 'well-refreshed' blokes got on at Prestatyn. One chap was recounting a recent remarkable experience and began with "So we were in a pasty shop, right, and we were having a pasty fight and....". I didn't actually catch the big exciting climax of the story - I'm guessing it was probably that somebody went in, bought a pasty and then actually took it away intending to eat it - as I was so open-mouthed at the having a pasty fight as clearly such a normal everyday factor, mentioned in passing for no other reason than to set the scene 