Kennedy is shown driving a car in England on the right side of the road. Cars are driven on the left side in England.
Both John Kennedy (as shown at home before going to the Pacific) and Leonard Thom are wearing the insignia of a Navy Lieutenant (O-3); at the time of the sinking of PT 109, Kennedy was a Lieutenant (Junior Grade) (O-2) and Thom was an Ensign (O-1).
When JFK goes back to the beach house to get his Navy uniform out of storage in the basement he walks past an Evian box in the basement. Evian (brand of drinking water) didn't exist until the 1980's.
The Kennedys are repeatedly shown playing on the ocean-side lawn at their house in Hyannis Port, Mass., with the sun setting over the ocean. Hyannis Port and the Kennedy compound face south, south east. The sun cannot be viewed setting over the ocean.