Brain fart

A brain fart is a colloquial term for that moment when a person thinks he or she has an answer or a solution to a problem, only to suddenly not know what the answer or solution is.

Empath: The Luckiest Smurf stories
In the EMPATH: The Luckiest Smurf story series, very few Smurfs call these moments "brain farts", but many of them can attest to the annoyances one can bring upon a Smurf at a moment of great importance.