Continuity mistake: When Emily and Whiff pull the rubbish trucks away, Whiff is coupled up to Emily, and Emily is coupled up to the trucks. When they pull them away, their buffers are separate.
Gordon and the Engineer - S11-E6
Continuity mistake: When Gordon is at the signal box, he drops off his coaches on his track and leaves. When he comes back with the mechanic, the coaches are gone.
Continuity mistake: The children waving at Toby disappear after Toby puffs under the bridge.
Continuity mistake: Alicia Botti was annoyed by Toby's appearance. But in "Thomas, Percy, and the Squeak", she thought Percy being dirty was the proper appearance of a steam engine.
Answer: He was in the water.