Revealing mistake: When the prisoners commandeer the balloon and proceed to take flight, you can see a television antenna on one of the buildings in the background. (00:07:20 - 00:08:00)
Revealing mistake: The sea creature Gorgo is paraded through the streets of London on a flatbed tractor trailer, and an off-screen American newsman announces the monster's arrival at Battersea Park, where it will be exhibited at Dorkin's Circus. The announcer introduces the creature's owners as they step from their motorcade, saying, "And our own Mr. Dorkin, of Dorkin's Circus, in the checkered suit." Problem is, Mr. Dorkin is wearing a plain gray flannel suit. Closeup shots of Mr. Dorkin over the next 40 seconds reveal that the suit is not checkered, not plaid, not striped, not patterned in any way at all. It's simply a plain gray suit. Apparently, the announcer's pre-recorded lines were never modified after changes were made in costuming. (00:34:05 - 00:35:00)
Revealing mistake: The Son of Hercules brings to the nearby village the old man that died on the beach. You can see the 'corpse' move his arm to assist Mark Forest as he dismounts him from the horse.
Suggested correction: You must have been watching a poor-quality copy of the movie. In the HD version available on Amazon, the checkered pattern is visible, although it is subtle. Frankly, it probably would not be visible on a television broadcast of the time.
I watched it in HD purchased from Amazon Prime on a large high-definition screen. No checkered suit.
Charles Austin Miller
You may need to adjust your settings. It is especially visible in the interview scene. The suit definitely has a checkered pattern of various shades of gray. Again, it is subtle, but definitely visible.
You may need to check your imagination.
Charles Austin Miller