Stupidity - either being seen as lacking intelligence, or pulling a stupid stunt, or saying something really dumb. Basically anything that results in people thinking "geez that guy's a retard."
When you know you only got the one ace, you're always scared of losing it. Not necessarily of it being beaten, since I'm rarely jealous or competitive with people about their intelligence. But falling short of an ethereal threashold value that makes people conclude "You? Smart? baahahahahhahahahhhahahahaahaha."
(e: this is one reason I broke up with the last girl I dated - I frequently felt stupid around her, and her constantly and sharply giving me a hard time, while very rarely indicating I was doing something well / right, only made it worse. She wasn't malicious, it was just an aspect of her personality that meshed wrong with mine, but motherfucker did it suck.)
Failure's similar for me, but it's because it leads toward this, rather than being a standalone thing.