Why am getting an "Image File Not Found" error?

I've gone through a few "Smush cycles" to process all the images on my site and am down to the last round-- currently the status bar says: 2565 of 2654 attachments have been sent for smushing." But when I try to send the final "Smush request" to process those last 90 file, I get an immediate "Image File Not Found" error. Why?