Factual error: A woman is shown receiving electroshock treatment ("ECT") in a hospital scene which took place in 1928. Although chemicals were used to induce seizures in psychiatric treatments at the time, the use of electricity for the purpose was not discovered until 1937 and not used in the United States until 1940.
Continuity mistake: When the "substitute" boy gets off the train he is with an older women. The reporters ask the mom and boy to take a picture together. You can see from the camera angle that the older woman is in the background when the flashes go off taking the picture. Later in the movie, the psychiatrist holds up a newspaper article with the picture in it. In the newspaper picture, the older woman is not in the background.
Question: Warning: Spoiler. At the end of the movie, it's found that three of the victims had managed to escape; only one was actually returned to the parents. It isn't known if the other two were recaptured by the killer or managed to get away (but went into hiding). Why didn't the police just go back and question the killer's accomplice? He'd likely be able to reveal whether any of the boys had been recaptured. (We're never told if the accomplice had died by that point, or whether he was unavailable, or whether nobody bothered asking him.)