The Sad Story of Henry - S1-E3
Revealing mistake: When the passengers try to pull Henry out of the tunnel, they have putty on their feet. Sir Topham Hatt's hand is also purple.
Revealing mistake: When Bumblebee is knocked onto his side, Ratchet kneels down to shove him back onto his wheels. The only problem is that when Bumblebee drives away, the animators forgot to add Ratchet's leg (that was previously obscured) into the frame.