Revealing mistake: As they're leaving together from Rosie's house at the end, Rosie stands up in the car to wave at people. The car sinks down with her weight when she puts her second foot inside the car, not when she takes it off the ground which is when all her weight would be on the car.
Revealing mistake: When the "real" Ralph is first shown at the Peace Corp office, he has an unattractive and nerdy appearance/voice. When arriving at Rosemary's party in the car, when saying "Amen to that", he suddenly looks fairly attractive and his voice sounds less nerdy. Then when saying "Good luck Hal", his voice becomes nerdy again.