Percival's father objects to his son "wearing a dress" when told by Mrs. Oleson that she has bought one for him (for his Christening, but she has yet to tell him that). Actually, boys of that era (1880's) were regularly photographed in dresses, so his objection is an anachronism, which no one would have voiced in the Victorian era.
Harriet offers Percival's mother a piece of chocolate cake. However, she should not have eaten it, because she did not know how Harriet greased the pan. If it was greased with lard, it would have been considered non-Kosher in the Jewish religion, since it comes from hogs.
Benjamin Cohen refuses to eat the roast beef Mrs. Oleson prepared because the meat isn't kosher, then proceeds to eat fruit and vegetables from the same (tainted) plate.
The Sabbath for Jewish people occurs on Friday night and ends on Saturday night, so they really weren't eating a Sabbath meal.
Jewish children are often named after a deceased relative, not a live one. The grandson shouldn't have been named after the living grandfather.
The name on the windows of Nellie's restaurant changes from "Nellie's" to "Caroline's" on several occasions.