In a 3 player Commander game, all humans, I cast
Villainous Wealth for x=43. The 43 cards were exiled from the target player's library. While I was choosing nonland spells to cast from the exiled cards, the targeted play quit the game, which removed all of his permanent's from the game.
Villainous Wealth still was waiting for me to choose cards from exile, but the exiled window no longer came up. I was able to click Done to finish my selection. The spells I had chosen resolved. I attempted to use
Dack Fayden's plus ability targeting me and a NullPointerException occurred. The game state rewound to before I cast
Villainous Wealth, including before I generated all of the mana using
Grand Architect and
Pili-Pala. The player that had quit was restored back into the match (he was still in the match chatting). I attempted to quit the match myself at this point because I didn't want to go through all the clicks to make that much mana again, but conceding match did not have any effect, so I had to exit the client. The gamelog seems to have not been generated unfortunately.
I suppose this was all caused by the player quitting and the spells staying on the stack. There is an interesting article about the things that occur when a player leaves a multiplayer game
here. The main thing being that all cards a player owns leave the game when the player does, including those still on the stack.