My favorite show of all time but some "bloopers"
Mr. Wilson referring to rare 1919-D dime, when in fact the rare one was 1916-D.
Margaret had 2 different last names (Wade, Harrington)
When referring to Georgiana in the wedding episode Mr. Wilson said "we are the only family she has left" what about brother John ?
When cat jumped in Mr. Wilson's trunk you can see man sitting in back seat crouched taking cat out. I think they thought he was hidden from audience.

As far as the Georgianna one ... that one is even worse when you take into account that we see two of Mr. Wilson's sisters in the first season, as well as Martha mentioning her mother and a sick sister, Edna. Personally, I am willing to excuse the blooper when it comes to John Wilson since his character was invented in a pinch and probably never would have "existed" at all if not for Joseph Kearns' death.
The Margaret one is funny. I think she only refers to herself as "Harrington" in "The New Principal," but I could be wrong about that.
Also, Sgt. Mooney has at least three first names! Ralph, Theodore and Harold.
Any more?

Margaret actually has three last names... she's the daughter of "Mrs. Moore" in the episode "A Quiet Evening".