In "Arthur's First Sleepover", Buster pulls out a deck of playing cards, and suggests they play a round of "Sloppy Sixes", but he never finishes explaining the object of the game because he misunderstood what Arthur meant when he asked "What's that?"
Is Sloppy Sixes an actual card game? I've never been certain, most people think it was made up for the dialogue instead of something else, like say, Crazy Eights.