Put it right after the claim and before the reveal, then follow the failed outcome with Awkward Crickets Sound Effect for the awkward moment that follows. If the mistake is dramatic, add Shocking Sound Effect next, or open with Yooooo Reaction Sound when the claim itself is already outrageous.
About this sound
John Cena delivers the question “Are you sure about that?” in a short, firm voice clip lasting about a second and a half. It feels funny and confrontational, a sudden challenge to someone speaking with too much confidence. The line is spoken by Cena himself and has become a familiar reaction sound in memes and short videos.