I can`t seem to export a project I`ve been working on forever. I keep getting an error

"Application failed to return a video frame. Canceling the operation"

What is the deal with that? I need this thing rendered out. I tried copying and pasting into a new sequence and that didn`t help. It did solve the "Error : Unknown Error" issue I was having, but now its not returning frames?

CS3 on an Intel Mac 2 gigs ram

Try creating a new project and importing the old one into it rather than copying and pasting. Sometimes works.

If that doesnt work...you could have a bad frame somewhere you need to defrag your hard drive

Did defragging REALLY fix a problem like this at any point of your career? I cannot imagine how fragged drive can lead to non-readable video. It can at most cause capture to drop frame, but the drive would have to be EXTREMELY old and slow.

I may be wrong, of course, but "defrag your hard drive" is the universal tip from Adobe, which usually doesn`t fix a thing.

Try searching Google "Application failed to return a video frame. Canceling the operation". You will find Microsoft Knowledge Base article describing this issue.
I had the same error message while exporting video to MPEG2-DVD in Premiere Pro.
It had something to do with Dual-core processor and a Windows patch fixed it.