The movie was released in 1970 and is set in that time period. As the flight approaches New York City near the beginning of the film there is a broad shot of lower Manhattan; The World Trade Center is not seen, so the footage was clearly filmed before the World Trade Center was built. The construction on the WTC towers began in 1968, and should be visible as the Kellermans overfly the site.
As George and Gwen pull over on the way to the airport to have a brief chat, it is clear they are in a country setting - the view out of the front window is of trees, grass, and foliage. But after their brief conversation, and without driving anywhere, they are on a busy street with a diner and businesses all around them.
Near the end of the movie, when George and Gwen Kellerman are flying home to Ohio, first you see a Boeing 727 in flight, supposedly the plane they are flying home on. The very last shot of the movie shows a Boeing 707 banking away from the camera.
When they leave the church George begins a long rant about not giving up. As he begins to step off the curb we see a line of traffic stopped at the light with a truck and a jeep being the first 2 in the line. As he continues his rant in the street the light changes to red and the traffic (now out of the picture) should be starting to move but there is no cars seen behind him. The line of traffic re-enters the picture still waiting at the light as he steps back up on the curb only now it is composed out of entirely different cars.
As the couple exits a congested Grand Central Station in New York, a very large "Notice of Filming" disclosure sign is clearly visible on the door George storms through to the street. Such signs are commonly used whenever a film crew shoots in a practical location, in order to indemnify the production from any recourse from shooting real people in public places, but they are normally far out of the camera frame.
In one shot in Central Park (as George and Gwen walk uphill into the trees just before they spot the crying Hispanic boy) and as they leave Central Park onto Central Park West, the shadow of the camera is clearly visible. In the earlier scene, the shadow follows with them until the camera turns and moves alongside them.
During the opening credits, scenes of NYC are interspersed with scenes that are supposed to be in suburban Ohio. The shots were of easily identified landmarks (clock tower, Gerry duckpond, etc) in Roslyn, NY.
When George & Gwen are dropped off in Central Park by the liquor store thieves, they open the rear doors of the police car from the inside. Police cars have their rear door interior latches deactivated and can only be opened from the outside.
When George finishes talking to the lost and found baggage clerk on the phone at the hotel desk he writes his name, Clifford Johnstone, down on a hotel sign-in paper on the desk which he leaves on the desk. He then argues some more with the desk clerk and takes down his and his manager's names too but he writes those names on a small note book he gets out of his inner jacket pocket and when he turns to leaves he puts the note book back in his pocket and walks away without picking up the paper that has Clifford Johnstone's name on it.
The goof item below may give away important plot points.
Two hijackers announce they are hijacking the aircraft to Cuba. Since the plane is on its final approach, there would be very little fuel left. The hijackers should have taken over the plane after takeoff in New York so the plane would have fuel to make it to Cuba.