Howard's age is miscalculated. Marion proves that today is Howard's 45th birthday by reminding him he was 31 when Joanie was born. Marion reasons that since Joanie is now 14, Howard must be 45 today. If all that were true, Howard would have already been 45 on Joanie's 14th birthday. Therefore, this day is his 46th birthday.
Fonzie states that his father left him when he was four years old, but, in "A Star is Bored," he says he was 12.
Howard: I thought you were Japanese?
Arnold: Only on my father's side. My mother is Chinese. See, I speak Japanese, Chinese, Korean, a little bit of Hungarian ...
Howard: Well, tell me something. What do you consider yourself?
Marion: Why would he just go off like this? Haven't I been a good wife?
Richie: Sure you have.
Marion: It couldn't have been my cooking. He was thin when I married him.
Howard: Oh, my goodness. I swore I was 43.
Marion: Well, it's not that much difference, dear.
Joanie: Yeah, my gym teacher told us: "Once you're over the hill, what's the difference how far?"