Revealing mistake: When Zack and Cody are tied up by the secret spy, when Cody is texting Bailey "Help Me" with his toes, you can see that the Mobile has a black screen, and is turned off. And he was just hitting random buttons with his toes.
The Scorn of the Star Sapphire! - S3-E5
Revealing mistake: While in the fight with Major Disaster, the double yellow lines on the road are wrong. They are too thick and close together. (00:12:30)
The Darkest Hour: Part Two - S4-E2
Revealing mistake: At the very beginning of the episode, as the knights are about to send Merlin to Camelot with Lancelot, the knights walk towards the camera. As they do, there's a patch of green screen the special effects department must have forgotten to remove between Sir Elyan and Sir Percival. (00:02:10)