Did You Check With the Captain of the Flying Monkeys? - S1-E6
Continuity mistake: When Alan is on the balcony talking to Evelyn, a shot is shown from inside the house. Alan turns away from Charlie and puts his left hand on his hip. The shot changes and Alan's left hand is now in the air and then he puts it on his hip again.
Chosen answer: Alan might have thought the bra was too fancy to belong to a regular woman, "I'd love to meet the girl who filled this", he says. He assumed it was one of Charlie's many women.
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