The Runaway - S2-E10
Visible crew/equipment: When Harold is flying, in one shot, a stage light is reflected off his blades.
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
Visible crew/equipment: When Thomas leaves the station, a tripod is reflected off him.
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.
Edward, Gordon and Henry - S1-E4
Visible crew/equipment: When The Fat Controller is looking at the goat, a hand is visible in the steam.
Edward, Gordon and Henry - S1-E4
Continuity mistake: When Henry comes out of the tunnel, he's dirty, in the next shot, he's clean.
Edward, Gordon and Henry - S1-E4
Continuity mistake: When the Fat Controller is asking Henry to pull the train, Gordon vanishes.
Edward, Gordon and Henry - S1-E4
Continuity mistake: When Edward is sad, the background behind him disappears between shots.
Edward, Gordon and Henry - S1-E4
Visible crew/equipment: When Edward is pushing the train, you can see a wire inside his four front wheels.
Edward, Gordon and Henry - S1-E4
Revealing mistake: When Gordon passes a signal, the track under him wobbles.
Visible crew/equipment: When Bill and Ben leave Boco and Edward, a tripod is visible in the top right (visible in HD full screen version).
Visible crew/equipment: When Bill and Ben pass a station, you can see a bit of studio equipment at the top of the screen. (Visible in HD full screen version).
Continuity mistake: When Bill and Ben notice their trucks are gone, there are guards in front of them, two shots later, there is no sign of the guards.
Other mistake: When Bill and Ben find Boco, there's a soup can on the left of the screen.
Thomas, Percy and the Dragon - S3-E7
Revealing mistake: When Thomas asks Percy about the guard leaving the light on, Thomas' right eye jitters.
Thomas, Percy and the Dragon - S3-E7
Visible crew/equipment: When James passes Percy, two studio lights and a crew member are reflected in James' paint.
Gordon Takes a Tumble - S6-E17
Character mistake: When the narrator says "but the signalman had accidentally left the points switched to the branch line" there is no switch, it's just a bend.
Gordon Takes a Tumble - S6-E17
Continuity mistake: The amount of trucks Gordon is pulling repeatedly change.
Visible crew/equipment: When Duck is being pulled out by Thomas, from the shot inside the barber shop, a camera and other studio equipment is visible behind Duck.
Answer: Breakdown trains help the engines back on track.