Revealing mistake: The old engine's model, who fell off the bridge, was actually Skarloey's, as some red paint can be seen.
Revealing mistake: A wire can be seen between Rusty and his coach after the workmen have removed the ice from Skarloey.
Revealing mistake: The waves hitting the ship are obviously being sprayed by a hose.
The Fogman - S6-E6
Revealing mistake: A piece of slate is already seen on the ground from a previous take before the slates hit the ground.
Revealing mistake: In a shot of Duck at the smelter's, a hole is visible on his running board.
Continuity mistake: Henry pushes Percy onto the coal tipper twice.
Plot hole: Percy was blamed for being on the coal tipper when it was Arry, Bert, and Henry's fault. None of the three got consequences. Also, why would the workmen tether Percy or even activate the coal tipper when they knew he was an engine?
James and the Red Balloon - S6-E15
Continuity mistake: When Percy arrives at the docks, he is carrying some freight cars. But when Thomas leaves the cars Percy was carrying disappear.
Gordon Takes a Tumble - S6-E17
Continuity mistake: When Gordon leaves the docks, he has his lamp, but in the next scene, it disappears.
Gordon Takes a Tumble - S6-E17
Character mistake: When the narrator says "but the signalman had accidentally left the points switched to the branch line" there is no switch, it's just a bend.
Gordon Takes a Tumble - S6-E17
Continuity mistake: The amount of trucks Gordon is pulling repeatedly change.
Audio problem: Mavis has Derek's horn sound.
Thomas, Percy and the Squeak - S6-E21
Other mistake: When Thomas sees Percy at the washdown, Gordon's front and rear bogie wheels are derailed.
Thomas, Percy and the Squeak - S6-E21
Continuity mistake: When Percy arrives at Brendam, his steam platform is visible and some of the steam from his funnel disappears, indicating a film cut.
Thomas, Percy and the Squeak - S6-E21
Other mistake: When Alicia the mouse is inside Clarabel, she is missing her forepaws.
Thomas, Percy and the Squeak - S6-E21
Visible crew/equipment: When Thomas arrives, his steam platform is visible.
Thomas the Jet Engine - S6-E22
Revealing mistake: After Cranky unloads the jet engine, it is already turned on.
Revealing mistake: Peter Sam is already missing his whistle even before it gets knocked off by the branch.
Visible crew/equipment: When Thomas says to Emily, "Give them back!", studio equipment is visible on the top-right corner of the screen.
Deliberate mistake: Due to the production order, Arthur can be seen at the quarry, but his introduction is 6 episodes later, in "The Spotless Record."
Answer: He was in the water.