The Backpack that Lewis is carrying during travel and to/from school is actually a real life Yugoslav People's Army Combat backpack Model 1975 - standard combat backpack from 1975 until 1990s and was also extensively used by every side during the Yugoslavian wars. It was uniquely designed around 1975 and not available in 1955. There are several varieties (black leather straps instead of leather colored and so on) and seems like front shoulder pads are removed, but definitely M75 - combat backpack of YPA.
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 items below may give away important plot points.
Florence and Jonathan believe Isaac's blueprint has been translated into an unknown language, until Lewis reads it using a decoder pin. However, any foreign writing (which uses a similar alphabet to English) still looks like written text, and nothing like the scrambled letters that are produced by a substitution cipher.
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.