At the beginning of the episode, Jonathan is driving as they discuss the nuclear disarmament talks. Then they break away to a scene of the car driving down the highway as they discuss what to do with their free day. In that scene, Mark is clearly seen driving the car. They must have swapped positions as they were driving.
At the fair the left pocket of Jonathan's leather bomber jacket is unzipped. But shortly after the right pocket is open and the left one is zipped shut.
At the amusement park, as Johnathan sends a balloon to the little girl after her original one broke, as she grabs it, the fishing line that's attached to the balloon allowing it to "float" in the air is visible.