"Not Found" mssg on rendering

Question

This is a new one on me.
When I use 'Apple R' to render, FCP behaves itself and does as it's asked, the file is rendered with no error message.
But if I use 'alt R' to render individual files, I get the progress bar as expected, but upon completion it's replaced by a window showing 'i' in a speech-bubble icon, with the words 'not found' beside it - and an 'ok' radio button.
There is nothing in the manual on this. The only references to 'not found' refer to external devices. My camera is hooked up as usual and working fine, as are the firewire drives.
The same thing happens when I allow FCP to do an auto render.
Anyone got any ideas why this has suddenly started happening?
Thanks
Tom

Answer 1

A little more experimentation reveals that this is only happening on one of my many sequences - but the problem is duplicated if I copy the sequence, or paste its clips into a different sequence (as I had done).
(All the other sequences seem to be rendering as normal).
This suggests to me that I have a problem file somewhere on that sequence which FCP either can't find or cant put away after rendering - though everything seems to be in place and playing correctly.
If so how do I go about finding the critter?
So far the project is just 10 hours of talking heads and a few FCP-generated captions - and I'm currently only removing, not adding, clips.
This whole sequence has played and rendered normally throughout the past two weeks.
Thanks
Tom