The Seven Wonders of the World Affair (1) - S4-E15
Revealing mistake: Illya flies a helicopter to the Himalayas in search of Solo, and at the same time, Kingsley's men are loading an unconscious Solo into another copter. Not only do the two helicopters look suspiciously similar - they have the exact same registration number: N73959. (00:44:40 - 00:46:30)