Occasionally, After Effects has a hiccup. When it does, it often displays a dialog box with occasionally clear, occasionally cryptic text, as well as a series of numbers. Understanding exactly what these are telling you can help you diagnose the problem and at least work around it, if not cure it.
Lutz Albrecht (aka Mylenium) has created a useful page that goes through the logic behind these codes to help you narrow down where the underlying error may be. He is actively trying to expand and refine the list, so if you encounter an error in After Effects, email him the information so he can fold it in.
(By the way, Lutz also wrote one of the best overviews of the new features in After Effects CS4 that I’ve seen – you can read it here.)