When Lamont first starts dating Janet, she has a brother but later in the series when Janet's ex husband returns Lamont then says Janet doesn't have a brother.
The exterior shot of the Sanford house/junk shop as seen in the opening credits does not match the exterior of the house/junk shop as it appeared on the show. In the opening credits shots, the house's front door is seen almost flush against the street with a very small front yard and little to no junk out in front of the house. In the show however, the Sanfords have a huge front yard with piles and piles of junk scattered about and the street is very far from the front door.
Both Fred and Lamont's birthdays change throughout the series. Sometimes Fred's birthday is in September and sometimes it's in December. Lamont's birthday is also mentioned as being in September and other times his birthday is either in January or February.
In "Ol' Brown Eyes" Fred has a television set in his room. Previously (and after "Ol' Brown Eyes" aired) the Sanfords only had one television set in their house. Also given how financially poor they were, it would have been impossible for a lower income family to have two television sets in their home.