Hopkinsville - S2-E4
Factual error: Dr. Hynek's wife dials a phone number to contact a club member. She dials four digits. Four digit telephone numbers were in use in the 1910s, and five digit numbers in the 1920s. By the 1950s, when this show takes place, there were even area codes and seven digit numbers already in use. (00:16:40)
Hopkinsville - S2-E4
Other mistake: Mr. Shoemaker tells Dr. Hynek and the Captain that the aliens he saw had three-fingered hands. A short while later he is caught giving a description of the aliens to a reporter. The picture drawn - based on Mr. Shoemaker's description - shows five-fingered hands. Plus later Dr. Hynek discovers a bio-luminescent hand print on a tree. It shows four fingers. (00:10:55)
Suggested correction: This is proved as a hoax so what the man said about the alien having three fingers could just have him mis-remembering the hoax details.
I can see that. OK.