Vassal Mouse with "play this now" Reactions can get really weird in the right (=constructed) Kingdom. With a Throne-chain type deck that relies on gainers rather than buying, you can basically take a turn in the middle of your opponent's turn. Add in Mandarin, along with Crown as your Throne variant and possibly Capitalism (which needs to have been bought by your opponent), and you can get your cards back out of play to be ready to do it again (and even stack your deck for your next Mouse Vassal "turn"). And if you happen to have a card that triggers your opponents' "play this now" Reactions, and they have similar decks...
Let's see: Caravan Guard, Rogue, Watchtower (Rogue+Watchtower=Mandarin gains not limited by the Supply), Crown, Mandarin, Storyteller, four other good Kingdom cards for this deck (preferably facilitating goals other than gaining cards, such as VP tokens, to keep the game from ending too quickly), Capitalism, and Way of the Mouse(=Vassal). And, of course, six players...
Note that while this could be an infinite loop, playing Rogue to gain Mandarin lets your opponents interrupt you with their own Caravan Guard Mouse Vassals, making it actually several infinite loops taking turns interrupting each other (assuming more than one player is set up correctly).