Wrong Road - S2-E22
Revealing mistake: When Gordon says, "I am Gordon! Stop! Stop!", his eyes are wonky.
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: Most of the engines' eyes are wonky when Diesel says their names and their eyes are wonky in a few shots of them.
Wrong Road - S2-E22
Revealing mistake: When Edward crosses the viaduct, the last coach of his train does not have lighting.
The Runaway - S2-E10
Revealing mistake: When Thomas' fireman is ill, you can see a brief flash of red outside, meaning it's James. (Note: same footage from Old Iron).
The Runaway - S2-E10
Revealing mistake: When Duck comes to Terence's field, the end of the track leads right into the bushes right behind duck.
The Runaway - S2-E10
Revealing mistake: When Duck is happy at the beginning, the breakdown train's buffer on the right is broken.
Wrong Road - S2-E22
Revealing mistake: Wires to power the lamps are very visible on Gordon's and Edward's buffers, and bulbs are also sticking out from both their lamps.
Answer: Breakdown trains help the engines back on track.