Factual error: After the plane is submerged, Jack Lemmon says, "... this plane is pressurized!" Jet aircraft are pressurized by bleeding air from the compressor sections of the engines into the cabin; in other words, the plane is only pressurized while the engines are running. Also, aircraft are only pressurized to a few psi above the outside air pressure at altitude, and never to more than the air pressure at sea level. The water pressure would be a great deal higher than the air pressure in the cabin, and since aircraft are not water tight, the water would quickly fill the aircraft. (If the aircraft was in 50 feet of water, the water pressure would be 21.7 psi, versus a maximum cabin pressure of 14.7 psi.)
Factual error: Donald Pleasence's character makes a left turn at an intersection controlled by a traffic light. The camera pulls back and reveals an Interstate highway shield on the road he turned from. Interstate highways never have at-grade intersections controlled by traffic lights; they are designed to be non-stop routes with access to side roads available only through ramps and grade-separated interchanges.
Continuity mistake: When Evel Knievel goes to the insane asylum to break his buddy Will out of the institution, he crashes through the front door with the Stratocycle, breaking off the airplane-style wings on each side of the bike in the doorway. In the next shot, as he rides down the hall, the wings are still intact.
Deliberate mistake: After the chopper drags the blimp about a mile out to sea, Kabakov unhooks it, and the blimp's bomb explodes seconds after. Why would he unhook it that close to shore with the dirigible's engines still pushing it back toward land and all those people? He had no idea when the bomb would go off. The bomb's fuse was set by hand inside the gondola.