Other mistake: The narrator says that Thomas' driver puts syrup in the lion, but actually, Thomas fireman puts the syrup on the lion. This is identified by his moustache.
Character mistake: The narrator says that Thomas had never delivered the mail before, but he did in Season 4.
Other mistake: Mavis and Cranky should've realised that Thomas woke them up the first time he blew his whistle.
The Biggest Present of All - S13-E16
Audio problem: When Henry and James are at the quarry, their whistle sounds are played out of order.
Continuity mistake: Thomas says that Mavis and Cranky never make mistakes, but they have in "Mavis" and "No Sleep for Cranky."
Answer: He was in the water.