We were a GM short for our last Deadlands session and so we played Something Went Wrong instead. It's a short, fast game, very light on rules: a group of experienced adventurers go on a last dungeon crawl to set themselves up for life. They have seen and done it all, so what could go wrong?
Character creations is done after assigning a class, an alignment and four stats. Oh, and a piece of loot, like the Jet-Black Dagger of Masterful Stabbing our group favoured. The GM sets up the encounter with the help of a random table and after one round of combat, passes the job of GM off to the player on her left. And just when everything is going really well, the GM has access to some more random tables with creative mishaps for everyone.
Our groupd had characters like Lootina the Thief, Cohen the Barbarian and the Great Fumblerado and fought against four dragons, four koalas and four tiny giants. People were roasted, tiny giants crawled up pants legs, koalas were clubbed to death with other koalas and a dwarf appearing out of nowhere served beer for everyone in an unexpected interlude. Somehow, everyone but two characters survived. One was killed, revived and then killed again when Cohen's axe slipped. Sorry about that.
If you're ever in need of a game for one evening that doesn't take a lot of explaining, is playable by a large group (we were six) and doesn't take itself seriously at all, check out Something Went Wrong. You can download it for free (or pay what you want) at Drive Thru RPG or at archive.org.