If your project compiles a bit faster than usual but doesn’t seem to work at all without even giving you any error. Or when debugging your project, it simply tells you that there is no actionscript code or something similar, you might want to have a look at this post first: http://blog.open-design.be/2009/05/27/5005unknown-error-when-optimizing-byte-code-bullshit/

And if this doesn’t solve your problem then you might have this problem:
Two symbols having the same Class name. I know, flash doesn’t allow that but when assigning a base class to a bunch of symbols, it might happen as it happened for me (I renamed a few symbols after having given them a class name). Normally flash tells you but sometimes it doesn’t…