That magic where it only blows up after deployment

lol environment variables are truly the standard go-to

If it works fine locally, you should start getting worried. That’s a normal sign.

If it was just a missing environment variable, consider yourself lucky... I once lost half a day because an old library version that was only installed locally caused an import error only in prod. There wasn't a single version pin in requirements.txt. Since then, I've switched to matching my local environment with docker-compose and running tests in CI by building the actual image. It's annoying, but in the end it's the fastest.