Florence tells Lewis that the Brothers Grimm wrote their Fairy Tales in the Black Forest (Schwarzwald) of Germany. In reality they wrote the tales in Kassel, which was a full day's journey from the Black Forest with the travel options of 1808.
Jonathan's last name is Barnavelt, the same as Lewis. But Jonathan is Lewis' mother's brother, and his last name should be her maiden name, not Barnavelt. The only ways they both could be named Barnavelt would be if her husband's name also just happened to be Barnavelt, or if the family used a naming convention that was extremely unusual and nonconforming by the standards of 1940s-1950s America.
The goof item below may give away important plot points.
Jonathan and Florence are frustrated to discover the Izards have been resurrected, and wonder who could have done it. But there are no other magic users in the town, apart from Jonathan, Florence, the Izards, and Lewis. It shouldn't have been a mystery as to who did it, through deductive reasoning and only a handful of magic users present.