Symfony Production Server Deployment Gotcha

Assume you did all the proper and mature building steps for your Symfony application. You have a build environment independent of your development environment. You really made sure all the environment variables like COMPOSER_NO_DEV and NODE_ENV are set correctly without getting the double negatives confused. And you execute composer, yarn, and whatnot with all the good switches like --no-dev, --frozen-lockfile, and --production. But you still get the following error message:

Attempted to load class "WebProfilerBundle" …