When Barney finishes his conga solo at Mary's, his hair is mussed. In the next shot, his hair is perfect again.
When Barney and the group are sneaking into Mary's house to surprise her and Andy, Barney opens the door from the outside with his right hand. But when they come in, he is opening the door with his left hand and holding a thermos in his right hand.
It's warm enough to ride in Mary's open convertible, yet there's a fire burning in her fireplace.
When the crowd is leaving Andy's house to surprise Andy and Mary at her house, the last two cars in the "parade" are being towed by the truck in front of them. A rope or cable can clearly be seen connecting the vehicles.
The Ford that Andy and Mary are sitting in at the lake has no windshield wipers and no rear-view mirror.