Best TV mistakes of 1966
Plot hole: Trelane says he studied Earth images that travelled to him at light speed, and earlier they establish Gothos is 900 years from Earth. But Trelane references Napoleon and Hamilton, who weren't around until 1800 or so. That would put this episode in 2700, but the original Trek episodes are set in the 2200's.
Audio problem: 1897: Quentin challenges Barnabas by pulling a decorative sword from a wall display and pointing it at his throat. Unfortunately, the display wasn't all that well attached to the set wall. Though the camera pans away, you can hear a loud clunk as the piece falls, followed by the loud crash of shattering glass as it strikes and breaks a table lamp. You can also see the actors struggling very hard to keep straight faces as they continue with their lines.
Add timeJean G
Continuity mistake: Phelps takes the tape from the recorder's right-hand spindle. When he opens the acid vat, we can see the tape he just removed back on the recorder - but on the left spindle. The tape he then drops into the acid doesn't remotely resemble the one he pulled, and when he drops it, the recorder's left spindle, where the full tape was in the last shot and where the empty reel should be, now has nothing on it at all.
Continuity mistake: Part 3: As you see the third "Sleeper" (a giant bomb shaped like the Red Skull's head) note the position of the monocle. First it's on its right side, then its left side, and back again to its left side.