I will review a board a dozen times, find no issues, submit it for production, then 5 minutes later discover an obvious bug. Characterize it how you will, the cancellation window has saved my bacon more than once.
Exactly this. You calm down, take a cup of coffee, marvel at your beautiful design, and then spot something out of place. The actual review has saved me a few times too, for example: "are you sure there are no copper layers in your PCB design?" - Doh! A few times they have raised issues regarding the limitations of their manufacturing capabilities, and this too has saved time and cost.