After Carl pulls Urkel back into the balloon basket, his left arm is either wrapped around the support bar or resting against it, depending on the camera angle.
When Carl falls on Urkel, they land on the couch at an angle that would have been impossible to naturally end up from where they fell.
At the end when Steve tells the Winslows he had to go home, he leaves in a hot air balloon, even though he just lives next door.
When Steve and Carl are in the hot air balloon and the plane is coming toward them, Steve says "Oh neat! A 727 is coming right at us.". A 727 is a three engine passenger plane with two engines on the wings and one on the tail, but the plane coming at them has four engines on the wings. The plane is most likely a 747 and not a 727 since they are the most common four engine passenger planes.