Failure, obviously. This ties in with my perfectionist habits, where if I don't do something I'm interested in perfectly the first time, then I see no point. It's something I've been working on for years and have gotten better at, fortunately.
That people might remember me not for who I am, but for who I was. I was (and still kind of am) the "weird" kid, and did a lot of stupid shit. Some of it was endearing, some not so much. A lot of the things I have said and done in the past have already come back to bite me in the ass, and so I'm terrified that one day someone might find out everything and everything I'v done up to that point was for nothing.
Death, specifically what's on the other side. I was raised in a conservative, Christian household (at least one that tried to be), and my parents talked about Heaven and Hell in the Bible like they were part of this binary, but given that people who could be wonderful non-Christians were also going to Hell in their eyes, I found it hard to believe. Granted, they never voiced that opinion very often, but it still bugs me, especially since I've started to question my own beliefs. Personally, I believe that there is a loving, omnipotent God out there, mostly because I think it would be strange for something as vast and complex as the universe to not be there one second and then suddenly exploding into reality the next.
Spiders. Fuck. Spiders. And stinkbugs, while we're at it!