Plot hole: Swapping his dental X-ray records with his deceased partner won't convince anyone that Varrick is dead - then (as now) the patient's name appears on the X-ray.
Audio problem: In the scene at the end where Joe Don Baker is chasing Walter Matthau's plane around the junkyard, you hear the unmistakable sound of tires screeching on pavement, yet the surface of the junkyard is all dirt.
Continuity mistake: Towards the end of the film when "Molly" is chasing Varrick's aircraft, the front fenders of his car have miraculously repaired themselves having been stoved in earlier when he crashed into various other cars in the scrapyard as he first went for the kill.
Continuity mistake: In the scene where the wimpy banker is talking to the boss near the fence, the shadows in the background vary tremendously. They start out as very small, then engulf the entire background. Obviously, hours were spent between takes.
Visible crew/equipment: As Charley is chased around the breaker's yard an entire row of people - presumably film crew - can be seen standing and watching over on the right. (01:39:26)
Character mistake: On finding Harman's dead body in their caravan, Charley says 'You called it, kid'. Great line, but Harman didn't call it. Charley, in fact, called it by giving his address to Jewell., who passes it on to Molly, who then goes to kill both Harman and Charley if he can.
Continuity mistake: In the bank robbery scene, Charley Varrick gets out of the yellow Continental, and closes the front passenger door behind him. In the next shot, the door is open. In the next shot, it's closed again.
Other mistake: When Molly heads to the van to kill Harman, we see Charley waiting in the scrub and watching the van, seemingly anticipating the arrival of Molly. But why would he do this? Does he randomly spend several hours watching for the arrival? Bizarre. Why not warn Harman? It's clear by his shock at seeing the dead body of his partner that it's not his intention to get Harman killed to get him out of the way. Seems to be an overhang of a previous plot direction where Charley does maliciously get Harman removed.
Revealing mistake: After Nadine Varrick dies, she is still breathing - her arm moves up and down in a regular, relaxed fashion, as if she is asleep.
Answer: That is a Boeing 'Stearman' biplane.