Place it at the reaction beat after the mistake lands, not during the action itself. For a bigger stack, follow it with the Emotional Damage Sound Button for the aftermath, or swap in the Fahhh Meme Sound Button when you want the same beat without the slowed treatment.
Über diesen Sound
The clip is a single drawn-out vocal shout of 'fahhhh' stretched to just over a second, with the pitch dropped and heavy reverb added so the voice trails off. It feels dramatic and mock-devastated, turning a tiny reaction into something oversized and funny, like a slow-motion collapse played for laughs.