Audio problem: Gordon had Duck's whistle sound and James had a different whistle sound in almost every Season 4 episode they appeared in.
A Bad Day for Sir Handel - S4-E6
Revealing mistake: When Sir Handel derails, you can see a small rod sticking up out of the ground from the tracks.
Peter Sam and the Refreshment Lady - S4-E7
Other mistake: In two shots shown at the beginning, Henry is wearing Gordon's upset face mask.
Audio problem: In the US version (Rusty Helps Peter Sam), Peter Sam has a different whistle sound after the trucks hit him. And throughout the episode, a lot of music and sound effects are missing.
Revealing mistake: When the passengers lift Duncan back onto the rails, his trailing wheels are still derailed.
Visible crew/equipment: As Peter Sam passes Skarloey and Rheneas, a camera shadow is seen.
Character mistake: On his way back to the sheds, Sir Handel goes through a red signal.
Audio problem: Thomas has James' whistle sound when he passes Sir Handel at the beginning.
Revealing mistake: Dried-up glue is visible around Rusty's face during the line, "Right, we'll find one tomorrow."
Audio problem: Stepney's whistle sound changes throughout the entire episode. And in the UK and other versions (excluding US), Gordon, Duck, and Douglas have the same whistle sound.
Continuity mistake: After Stepney's driver tells Stepney to speed up, Caroline is nowhere to be seen.
Bowled Out - S4-E18
Continuity mistake: When the diesel grinds to a halt and the camera switches to Duck and Stepney and then back to him, he is noticeably further away from the coaches.
Bowled Out - S4-E18
Audio problem: When James leaves Crovan's Gate, he has Smudger's whistle sound.
Revealing mistake: In a shot of 2 vans singing "Pop Goes Old Ollie" you can see a spider crawling on the right side of the left van in the screen.
Answer: Breakdown trains help the engines back on track.