Put it at the end of a failed attempt, after the mistake is clear, and use Dun Dun Dun before the reveal, then close with SpongeBob Fail if the flop keeps getting worse.
About this sound
This is a short vocal wah wah wahhhh sound, lasting about two seconds, with a drawn-out, descending wail that mimics a sad trombone. It gives a listener an instant sense of comic disappointment, like a joke landing flat or a plan going wrong in front of everyone. The exaggerated, drooping tone makes a small failure feel bigger and funnier, turning an awkward moment into a shared laugh.