What Error Appears in the Scene Where Cary Grant Is Shot at by Eva Marie Saint in North by Northwest (1959)?

The shooting in North by Northwest (1959) is supposed to be a surprise to onlookers.

But in the background, a boy extra (who has rehearsed once too often) puts his fingers in his ears before the gun goes off.

• What Error Appears at the End of the Invisible Man (1933)?
• What Was the “Old Northwest” In America?
• How Many Wives Did Cary Grant Have?
• How Many Times Did Ralph Bellamy Play the “Other Man” With Whom Cary Grant Had to Compete for the Heroine’s Affections?
• What Was the Name of the Dinosaur Bone Lost by David Huxley (Cary Grant) in Bringing Up Baby (1938)?