Fergus Breaks the Rules - S7-E18
Revealing mistake: A wire is under Fergus when he arrives at the smelter's.
Revealing mistake: When Murdoch asks Gordon "What decorations will you have, Gordon?" his left eyebrow (viewer's right perspective) is scratched.
Revealing mistake: When Toby tells the Fat Controller about the broken windmill, his driver's head appears to have been digitally added.
Answer: He was in the water.