Continuity mistake: Hannay is talking to the milkman inside the door. Outside the door you can see across the street, the two guys walking down the sidewalk. A moment later, Hannay goes toward the door and now there's the milk wagon on the street right in front of the door. When Hannay pointed out the two guys across the street to the milkman, the milk wagon wasn't there and the milkman is inside with Hannay so he couldn't have pulled it up.
Continuity mistake: For the early happy ending, Ellery Queen takes the jeweller by surprise giving him the pearl necklace, and the fella, incredulous, touches his breast. The hands swap position at the first cut. (00:07:25)