Revealing mistake: Salty is missing his rear lamp in the first shot at the docks and a hole is visible where it should be.
Revealing mistake: Dried-up glue is visible around Rusty's face during the line, "Right, we'll find one tomorrow."
Revealing mistake: When Toby asks Thomas if he took on enough water, Toby's eyes are wonky.
The Sad Story of Henry - S1-E3
Revealing mistake: When the passengers try to pull Henry out of the tunnel, they have putty on their feet. Sir Topham Hatt's hand is also purple.
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.
Revealing mistake: Max's wheels can be seen moving backwards when he is actually driving forwards. Also, both Max and Monty's steering and wheel speeds are out of sync with their movements. These events happen throughout the episode.
Santa's Little Engine - S17-E18
Revealing mistake: At one point, the shading effect on Thomas' whistle is not rendered properly, so it appears silver instead of gold.
Revealing mistake: A wire can be seen between Rusty and his coach after the workmen have removed the ice from Skarloey.
Thomas and the Breakdown Train - S1-E7
Revealing mistake: In the scene of James' wheels sparking while passing Tidmouth Sheds, a metal rod is igniting the sparks from his wheels.
Revealing mistake: When Murdoch asks Gordon "What decorations will you have, Gordon?" his left eyebrow (viewer's right perspective) is scratched.
Revealing mistake: When Duck goes off to fetch Gordon's coaches following his chat with Diesel, the device beneath the tracks used to produce steam continues to smoke long after Duck has left.
Revealing mistake: In a close up of Percy's driver, he is in Henry's cab.
Revealing mistake: When Percy is at the "backing" signal, his cylinder is sagging.
Revealing mistake: When Edward is at the last station, his whistle is crooked.
Revealing mistake: After Gordon stops on the bridge for the cow, his wheels keep moving for 3 seconds.
Revealing mistake: When Percy stops on the bridge, white glue is on his middle lamp iron.
Revealing mistake: Edward has wonky eyes in some scenes.
Thomas, Percy and the Coal - S2-E1
Revealing mistake: In some shots of Thomas and Percy, both engines have wonky eyes.
Thomas, Percy and the Coal - S2-E1
Revealing mistake: When Toby pulls Annie and Clarabel, the latter coach has a loose roof.
Revealing mistake: Each time when Thomas takes a shortcut, the points do not change.
Answer: Breakdown trains help the engines back on track.