I just had this happen to me on an edit! Since it still played, I just finished the edit and all was fine. However, it was only the thumbnails in the timeline that showed offline and not on the bin. I suspect it’s the thumbnails that became corrupt. I didn’t try this, but you might be able to delete the thumbnail cache, while FCP is closed, then when you re-open FCP, it’ll re-populate the thumbs. Lemme know if it worked!
Tony Brittan
Apple Certified Pro – FCP
Islandshoreproductions.com