Mrs. Myerson's voice in the two phone calls sounds different from that of actress Anne Haney who plays the part in a later scene.
In the scene where Kendig is chased by FBI cars in Georgia and spills oil on the road, it is shown as he left it and again a moment later as the FBI cars careen off the road. The splash pattern of the oil on the road is different in the two views.
The movie is plausible up to the point where Matthau was able to transition from the biplane, across an open field under the watchful eye of four people in a helicopter without being seen, to hide in the shack with his radio control box and fly the airplane. Inconceivable.
When the agents from the CIA and FBI got near Kending's boss house, there's his car in front of the house, when they start shooting the car disappears.
There's a scene at Myerson's house in Savannah where Kendig is getting ready to start typing and sees a photo of Myerson. When the photo is first shown, Myerson is facing to the left of the screen, but when Kendig picks it up Myerson is facing to the right. The photograph changes several times later. In fact, each change is intentional, and is intended to show Myerson getting more frustrated and dour as the plot wears on, culminating in the picture being shot in the forehead (as explained in the introductory video on the DVD).
When Glenda Jackson leaves her house in Salzburg in her car "to get some ice" it shows her driving instead to a ferry boat which takes her to England, apparently arriving later the same day. The distance from Salzburg to the English Channel cannot be driven in one day.
Myerson's house is supposed to be in a suburb of Savannah, GA, but the address is stated as being in Adairsville. Adairsville is north of Atlanta, more than 300 miles north of Savannah.
Miles mails his first chapter to the CIA in Washington, D.C. - but the agency's headquarters are in Langley, Virginia.
Agent Ross's passport shows a birth date of 30 January 1930, but actor David Matthau was obviously much younger (born 1953).
When Kendig puts on his reading glasses in Myerson's office to examine the photos on the wall close up, he looks over the glasses rather than through the lenses to view the pictures. There's no point in putting on glasses if you're just going to look over them.