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.
Continuity mistake: The package Steve received from Mailbox is briefly visible before Mailbox comes in. You can also see a crew member's hand holding the package as Steve grabs it.
Blue Wants to Play a Song Game - S1-E14
Revealing mistake: After Blue and Steve play the monkeys jumping on the bed game with the sock monkey, when the mail time song plays Steve gets up excitedly and his arm and hand get cut off.
What Does Blue Want to Make? - S1-E15
Visible crew/equipment: A boom mic is visible when Steve starts singing "We Just Got A Letter."
A Snowy Day - S1-E10
Revealing mistake: During the mailtime segment, when Mailbox shakes snow onto the chair, a small part of him is cut off, and you can see the letter being held behind him. This also happens when Mailbox opens his door, and you can even see a crew member's hand move away as Steve grabs the letter. (00:17:38)
Snack Time - S1-E1
Visible crew/equipment: After Steve skidoos into the farm, the boom mic dips into view above his head. (00:15:46)
A Snowy Day - S1-E10
Other mistake: When Mailbox comes in, he shakes some snow onto the chair. When Steve brushes it off, the snow stays visible on screen, then disappears mid-shot. (00:17:43)
What Is Blue Afraid of? - S1-E18
Other mistake: Steve finds the second clue, which is the shadow of an owl. The owl is on a tree branch, but there is no tree shadow that the branch shadow is attached to. A floating tree branch? (00:06:41)
What Is Blue Afraid of? - S1-E18
Other mistake: Shovel, Pail, and Steve see a shadow, then figure out it's the shadow of a tree. Then, when the red bird comes and lands on the tree branch, the bird's shadow is reversed, with the tail facing the opposite direction. (00:05:58)
What Story Does Blue Want to Play? - S1-E16
Revealing mistake: When Blue wants to play Blue's Clues, the paw print is off-model.
Revealing mistake: The first clue, the boots, are cut off to the left.
Snack Time - S1-E1
Revealing mistake: When Steve skidoos back to the thinking chair, the tree branch is cut off.
Snack Time - S1-E1
Revealing mistake: When Steve skidoos to the farm, there is a white border around the screen.