The Runaway - S2-E10
Audio problem: When Thomas is tired and approaching the station, he is moving pretty slow, but the sound of Thomas' wheels moving is like he's going pretty fast.
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.
Answer: He was in the water.