Yellow triangle error in Final Cut Pro library

I've tried similar solutions lick copy and pasting the Project with success. Another thing to try is to make a compound clip of the entire timeline and then make a new project using that compound clip. Agreed that it is very odd. I have tracked down the offending issue by copy/pasting the entire clip into a new timeline for safety sake, then deleting half of it and attempting to export, knowing if that works it's in the other half...on at least one occasion it was a tiny bit of transition that somehow got left between clips and was virtually invisible until the timeline was blow up for close inspection. It's def an issue that FCP should seek to solve so we can find the problem more easily.

Maybe the word "missing" is not exactly what you need to search for.

Clear the search text, and go through the index. Is there any yellow warning there?

Try this: in the problem project, Command-A (select all), then Command-C (copy)

Make a new project with the same settings, and Command-V (paste)

Does the new project show any errors? Does it export?