Continuity mistake: West tosses a small grenade at the French doors. We see it hit and explode without breaking a single pane or crosspiece. It just makes smoke and blackens the surface. After a shot of West and Gordon, we cut back to the door and it's now a broken ruin with no glass and the wood framing destroyed. Delayed action bomb?
Factual error: Artemus opens a cabinet to reveal a map of the United States where he and West are keeping track of the alphabetical murders. The map is far out of date for the Grant administration, with what became the state of West Virginia during the Civil War still part of Virginia, and missing entirely the states of Nevada, Oregon, Kansas, Nebraska, and Minnesota-all of which were part of the union before President Grant took office.