I was getting a white screen with 200 status in production after updating Symfony to 3.4. There was a silent error, “Cannot declare class appProdProjectContainer, because the name is already in use”. Not sure why the error was getting swallowed or why a normal cache clear didn’t handle it.
rm -rf var/cache/prod/* fixed it. Not part of my normal deploy, though maybe it should be.
The Symfony part of my site was down for a day, since dev was working fine, and I forgot to check prod.