Corrected entry: On the globe April uses to show their kids where they are moving, Germany is not separated into western and eastern part. Thus, the globe is post-1990 equipment.
Corrected entry: OK, this is arguably not an error, but here goes: The car Frank drives in 1955 is a brand new (in the year of the film), spotless 1955 Buick Roadmaster, their most expensive, largest 4 door sedan. The car is missing an essential body part that would never be missing from a brand new car: There are no fender skirts.
Correction: Never start a mistake with "This is arguably not an error". If even you aren't sure about it, then you have no business submitting it. In any case, this is not a mistake.
Correction: Or pre-1949 when Germany was divided. In that case, not a mistake.