Thomas Tells a Lie, sometimes known as Thomas Breaks a Promise, is a book.
Thomas is sent to check signals on a new branch line, but he becomes distracted by a sign for a carnival and forgets. Percy has to use the new line to deliver the mail that night because the other lines are flooded. One of the broken signals causes Percy to derail his train. Thomas confessed next morning, and Sir Topham Hatt makes Thomas check every signal on his railway twice. Then Thomas and Gordon check the signals.
- The book was featured as a read-along bonus feature on both, Percy's Chocolate Crunch and Other Thomas Adventures and Thomas Gets Bumped and Other Stories DVD, narrated by Robin Smith.
- When Thomas stopped at the red signal the points crossed over. They are supposed to join.
- Gordon is too heavy to go on branch lines.
- Toby's funnel is missing.
- James' splasher is not coloured black.
- Percy's coal bunker is green.
- Gordon's tender axle boxes are coloured blue.
- Percy's headlamp changes position.
- James' wheels and tender axle boxes are coloured grey.
- In the fifth illustration, Thomas' back buffers are missing.
- According to the text, Percy saw an unlit signal lamp. However, the illustrations do not show any lamps on the signals.
- In the illustration when the engines are gathered, Gordon and Henry's funnels are too tall.