A Weekend in the Country - S3-E3
Revealing mistake: When Orson and Andrew are sitting in the diner, the same man in the green shirt keeps passing the window.
What Would We Do Without You? - S3-E22
Revealing mistake: When Lynette tears open Tom's shirt before making love to him, you can see there are already scratches on his chest before she scratches him.
Chosen answer: Poker. They have been playing that this whole time, once a week with everyone who was invited that lived on the Lane.