1. What happens on a MeFi Meetup stays at the MeFi Meetup
2. what happens at MeFi meetup stays at MeFi meetup
3. what happens at Meet-up stays at Meet-up

Now, “meetup” is (in normal use on metafilter) a common noun that takes an article. So form (1) is the only unimpeachably grammatical of the three.

But Vegas, of course, takes no article and is a proper noun, and so (2) and (3) hew more closely to the strict form of the snowclone template itself, even if they require a bit of stretching from conventional use of “meetup”.

So it’s interesting to see examples of people making that decision in either direction.

Also interesting is the use of two different prepositions in (1): on, and then at. I’d say I normally go to a meetup and spend time at it, but I suppose someone could consider a meetup like a field trip and thus something you can go on. Especially so if it involves travel or a group excursion, I suppose.

(Most meetups generally focus on group libation.)

And what you say in chatfilter, stays in chatfilter.

That’s exactly the sort of thing that I think qualifies as a specimen of the snowclone yet which is very hard to search for by normal means. The more generalized form there could represented like so…

What V in X stay[s] in X

…where V is some sort of verb phrase of which “happens” is only the canonical form. It’s hard to search for here because, as I mentioned in the blurb of the “X, Y” section above, there’s not a lot for a search to latch onto, if I don’t have a specific idea of what X will be, because all I have to go on are the words “what”, “in”, and “stay[s]“.

I may exploit my access to the mefi db to do a more structured string search at some point so I can do a proper digging-about for the ordered “what * in *[,] stay[s] *” template, but even then I’m sure there’s variants I’ll miss.