Riff Amnesia happens when you go into a musical instrument store to try out amps and guitars but once you get down to the actual playing of the guitar you loose all memory of the riffs you know and everyone within earshot thinks you suck.
Riff amnesia often makes the sufferer feel vastly inadequite to the guys who peel off ball tearing solos at the shop to show off.
"Man, I went to Dinny's to try out that new Les Paul Custom but got Riff Amnesia and couldn't play anything. The guy in the next room started playing Metallica solos to show off. I felt like a fool."