I perform horribly under stress. My biggest problem occurs when I make a mistake; once that first mistake is made, it just breeds more mistakes, and then I'm totally screwed. It does not matter whether or not I was doing fine or already stressed when I made the first mistake. That is why I try so hard to be perfect. I am completely incapable of making just ONE mistake.
It's especially bad if I am around someone who doesn't know me well, because their impression of me is that I'm an incompetent idiot.
And, of course, when I'm under stress, that's when all those lovely AS traits rear their ugly heads, making the problem even worse.