Factual error: The Liberty Bell is depicted ringing as the Declaration of Independence is being signed on July 4 (a mistake itself) in the film. The bell actually did not ring on that date, but on July 8, when the Declaration had been returned from the printer and unveiled to the public.
Continuity mistake: Fiona Fullerton's eyebrows change from full to plucked and back again all through the film.
Continuity mistake: When Tintin and the two kids are escaping from the submarine lair, the water level keeps changing. It's supposed to be more and more high, but it's lower in some shots.
Continuity mistake: When Woody enters the café and sits at the table, Miss Meanie pours him a glass of water. The glass disappears in the next shot.
Continuity mistake: When Mr. Kumagoro visits the Uncle at the hospital, he drops a package on him; the niece looks at him startled, her arms crossed in a different position between shots.
Revealing mistake: When Anguirus makes his sacrificial charge against Gigan, the hand that is moving the Anguirus costume can be seen holding it.