Mr. Brown, a butcher who should know better, authoritatively announces that one small-to-medium-sized lobster will provide a family dinner for six: A claw apiece for he and his wife, the entire tail (the meatiest part of the crustacean) for his picky young son who's a finicky eater and what's "in-between" (basically all of it's inedible entrails) for his three teenage daughters - with some left over for the dog! In reality, a lobster of that size would hardly feed one hungry person.
Incorrectly regarded as goofs
The story takes place in the late-1920s, and the Browns pay the entry fee with "100 gold sovereigns" which ceased to be used as money in England in 1918. However, this is consistent with a detail from the novel, in which Mrs Brown insists the entry fee be paid with the sovereigns she won swimming the English Channel, which occurred prior to 1918.