For Sach to see Count Boris Petrov staring at him through the bedroom window, Sach must first wipe away the interior condensation on the glass; but, when Teresa and Terrence Mahoney Esq. see the count, it's the count who must wipe away grime from the exterior of the glass.
When Gabe walks up to the Mahoney Manor door, his hands hang freely, but when a different camera takes over, his hands are in his coat pockets.
When Terrence Mahoney Esq. yells in fright from seeing a man staring at him through the bedroom window, extra yells are added, revealed by his mouth not moving to utter them.
The boys live in the Bowery section of Manhattan, New York City, but the building they are shown working in is the Shell Building in San Francisco, California - although they have "Metropolitan Building" on their work coveralls.
Near the end of the film, as Slip ascends the stairs lamenting his loneliness, a moving shadow of the boom microphone appears briefly on the wall to the left.
When Captain Drum rows the smugglers into the cave, he makes a very lame attempt to moor the boat to a post. The rope unravels and falls into the water as he walks away, risking the rowboat floating away. A real ship's captain would not fail to tie a stable nautical hitch.