When Paris (in disguise) and the real Emil Vautrain are in the same room, Vautrain passes behind Paris (seated in a chair) in the same shot, even though both characters are played by Leonard Nimoy. It is clear that a stand-in is posing as Paris for this shot, as his fake nose is a bit grayish.
When testing the voltage on the electric chair, the voltmeter reads only 1000 volts, even though Phelps claims it's 10,000 volts.