Really its just when some of the players can't show up for the main game or the GM is feeling too tired. What we do is well make up something. Most of the time we make up some kind of simple rules and we just try anything. The only real rule is everyone has to be OK with it and to have fun.
In the past we've done police procedural/horror “Night Shift” post apocalyptic unfrozen convicts “Chosen Frozen” violent action “Hit Squad” and even with a new player never before GM'd game of Avatar the Last Air Bender “Three Elements Team Are Go!”
Rule have been all over the map, d6 systems, d20 1-4 Crit Fail, 5-16 Succeed 17-20 Crit Succeed, 3d6 GURPS like, d10's whatever's handy.
And you know, it works really well.
Why it does is not a mystery.
We have highly functioning high trust players. No Dinner Table Knights but just good solid gamers willing to lead and follow and help each other out.
This allows us the freedom to experiment, the ability to quickly end bad games (Faerie Cafe was not a success ) and the ability to trust both GM and Player to work together to bring on the fun.
And believe me not every group has that.
You see ,the default assumption Gygax and Arneson, all full grown mature adults had was that gamers were also imaginative mature grownups and in the 20 and 12's that is not always the case.
And no I am not talking about the games we played at 13 and 14 (Gor themed D&D shudder) or as kids, at that age a little juvenile behavior is to be expected.
No , I am referring to games where the players are theoretically adults. And I mean theoretical . Unfortunately many “adult” players these days have issues and those players, the immature, the self centered and the emotional runts can ruin any game.
They make high trust games, old school games difficult and suck up the fun with their issues and drama. Yuck.
Being there are so many, you can't always spot the or ban them much as you want to, you adapt and you play like B.A. Felton at least till its not fun, than its back to X-Box.
Happily however my current group while younger by a decent measure than me or the original founders were , are all adults .
But by dint of them having had had responsibilities and good parental investment they are as grown up as they should be for their age and as people were back in the founding era of the hobby.
That means they can do Old School , New School, our fave GURPS or Robot Chicken Night with aplomb which means more fun for all us.
Thats wonderful and having been exposed to a lot of bad gaming I am so aware how great it is to have a group like that. And so if any of y'all are reading this, thansk for being awesome.