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" from namespace "Symfony\Bundle\WebProfilerBundle".
Did you forget a "use" statement for another namespace?
Then you might have forgotten to set APP_ENV=“prod” on the production system. It of course makes sense when you think about it. But it always does, once you figured out. Anyway: I hope this helps a fellow traveler at some point in the future! 
Article Link: Symfony Production Server Deployment Gotcha – nullteilerfrei