According to the second season Halloween episode "Haunted", Fonzie had his tonsils taken out when he was a child.
In the beginning, when Fonzie stands up to say he us not going to the hospital, you Joanie, Richie and Mr C standing in that order, but when Mrs C starts setting Fonzie straight, Mr C and Joanie, has switched spots.
One of the kids in the hospital room that Fonzie is staying in on Halloween is wearing a Ben Cooper brand Incredible Hulk mask. The Hulk as a character wasn't created until 1962, and that particular mask came out in 1976.