Much is made over the fact that Dr. Melton, the murder victim,
was left-handed. Yet, earlier, he was seen doing various things, such as taking notes, opening a door, and turning on the fatal light switch, all with his right hand.
The mounted gargoyle head in the scientist's lab is able to swivel, following Birmingham's movements due to photo-electric sensor, yet when knocked over in gunfire, it's shown to be constructed on a simple wooden mount (a pole with a circular base) with no wiring involved whatsoever.
The electromagnet used to fire the gun, did not affect any other metal objects in the room.
The prop for the electromagnet was a toilet float ball attached to a lamp.
Although set in Washington D.C., in the first exterior shot when Charlie Chan leaves the office building you see the California state flag outside.
While Charlie Chan is riding in a taxicab to the murdered doctor's house in "Washington, D.C.", several exterior shots show the taxi driving through a neighborhood full of houses with red tile roofs and palm trees in the front yards... similar to that of West Los Angeles, California.