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.
During the chase scene by the mounted policeman across the sheep meadow George and Gwen are shown crossing a paved path. There is no paved path in the middle of the sheep meadow.
Several characters repeatedly mention the New York City area being fogged in (seriously enough to disrupt transportation) but there is little or no sign of fog during any subsequent exterior shots. In addition, after experiencing heavy rainfall overnight (a "monsoon" in one character's description), Central Park pavement is remarkably dry just a few hours later.
Two characters on foot would not be able outrun an officer on horseback.
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.
While in South Station in Boston a train announcer is heard announcing a special train number 15 to Saint Louis. The announcer can be heard listing stops the train will make en-route such as Poughkeepsie, Albany, Schenectady, Utica, Syracuse, Rochester, Buffalo, Cleveland, Indianapolis, and Saint Louis. The mistake here is that the New York Central trains that operated Boston to Saint Louis would not pass through Poughkeepsie. After the Kellermans arrive at Grand Central Station in New York that same train announcer can be heard announcing that same special train number 15 and the same stops.
In the police station scene, the desk sergeant is seen talking on the telephone but none of phone line buttons are lit (indicating an active call). This is made more obvious as no lights go off when the headset is returned to the cradle.
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.
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.
When George and Gwen Kellermen are walking in the rain to Waldorf Hotel and Gwen stops to look at garbage on the street, a thick microphone wire is visible extending from George's suit.
In one scene in a hotel room, the motion picture camera is momentarily visible in a reflection in a mirror hanging on the wall.
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.
When George and Gwen lose their luggage at the Boston airport and run around trying to look for it, camera shadows are visible throughout this sequence.
When Jack Lemmon asks Sandy Dennis if she is going to stand forever on the corner of 65th Street, they are actually on the corner of West 68th Street and Central Park West.
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 the Kellerman's finally get into their hotel room, George embarrassingly tells the bellman that he's out of cash except for four pennies which he's perfectly welcomed too. This is due to the Kellerman's being mugged and thus loosing all their cash. However, Gwen earlier stated that she packed some extra cash in their little gray suitcase. The gray suitcase, which does not have a lock on it like their larger brown suitcase, is just sitting their. If either of the Kellerman's had remembered that fact they would of realized that they are no longer bereft of cash.