Revealing mistake: In the intro of this episode, a glitch is visible.
Blue's Clues (1996)
1 revealing mistake in season 3 - chronological order
Starring: Aleisha Allen, Steve Burns, Seth O'Hickory, Traci Paige Johnson
Blue Wants to Play a Game - S1-E12
Other mistake: The jigsaw puzzle Steve and Blue help Shovel and Pail with has a picture of everyone playing musical chairs from the Mailbox's Birthday episode, but Steve suddenly isn't in the puzzle. He played musical chairs, so he should've been in the puzzle.
What Story Does Blue Want to Play? - S1-E16
Trivia: At the end when Steve sings the So Long song, he's impersonating Elvis Presley.