Continuity mistake: While they're having dinner, Johnston spears several carrots on his fork, nudging one off the edge of the plate and onto the table. In the next shot, it's gone.
Deliberate mistake: The mystery begins as Winston points out the postmark on the envelope that Dan receives, it is too faint to see where it's from. But whenever we see a close up of the envelope, there is no postmark at all.
Revealing mistake: When the phone rings in the motel room, the red light on the phone does not blink which is how those phones work.