Originally, Klaus was going to be French, and named François. When Dee Bradley Baker auditioned, he told the producers that his French wasn't very good, and that he would read the lines in German instead, they liked his performance so much, they retooled him so he was German instead.
Steve's original design was much geekier and gawky, and he was voiced by Ricky Blitt. Halfway through production, his voice actor changed to Scott Grimes, and his design was made to be more attractive, so he would be less comparable to Neil Goldman, from Family Guy (1999). This design is a caricature of Series co-Creator, Mike Barker.
Hayley says in American Dad!: Stan Knows Best (2005) that she is eighteen years old. Based on this information, we can assume that she was born in either 1986 or 1987 (depending on her month of birth).
In "Meter Made", Francine asks her sister Gwen "how are you enjoying being three years younger than me". Later, in "Now and Gwen", Gwen is a senior in high school while Francine is a freshman, making her roughly three years older.
The uniformed officer, in the Underdog Productions logo at the end, says something different in every episode. (Not unique, just out of a small assortment.) In the season seven finale, he was given a speaking role as Steve's imaginary friend "Pedro".
The final frame of the introduction was changed for seasons beyond the first. At the end of the credits in the first season, all that could be seen was a big "American Dad!" logo in front of the American Flag. In all subsequent scenes, this logo was shrunk to fit the names to the series creators on the same screen.
In the season three episode "Camp Refoogee" Stan flies to Africa and turns a refugee camp into a summer camp. However, in the season seven episode "Ricky Spanish", a sponsored child from Africa, named Tungee, comes to live with the Smiths. Stan asks, "Does this mean Africa's real?"
From the season five episode "May the Best Stan Win", it was revealed at the end of the episode exactly when Stan and Francine were born. Stan was born in 1967, and Francine in 1971. We see this information written on their gravestone when they died and are buried next to each other. Today, 2017, Stan is fifty years old and Francine is forty-six years old. Both will die in 2067, Stan will be one hundred years old, and Francine will be ninety-six years old.