Continuity mistake: The narrator says that Dart had never seen the logging locos before, but he saw them 3 episodes ago.
Character mistake: Throughout the episode, a "Bufferbeam" is referred to as "a buffer." Also, at the beginning, the narrator says that the diesels are shunting coal trucks when they are really shunting flatbeds.
Revealing mistake: In a shot of Gordon arriving at Brendam Docks, only half of his tender is visible.
Thomas and the Sounds of Sodor - S16-E11
Deliberate mistake: When the engines are performing "The Sounds of Sodor", Emily, Thomas, Percy and Gordon have the correct whistle sounds but at a lower pitch. At one point, James has Emily/Henry's whistle sound, Mavis has her old "New Series" horn, and Thomas has James' whistle sound.
Continuity mistake: When Thomas hides behind the trucks, there are only 3. But in the next shot, there is a flatbed with a crate on it in front of the trucks.
Answer: He was in the water.