This is part of the Semicolon&Sons Code Diary - consisting of lessons learned on the job. You're in the dumb-mistakes-and-gotchas category.
Last Updated: 2024-04-25
If you cannot find some code or some command line option that is causing a bug or appearing in an error/warning inside the source code of the project or library, then search within the source codes of its dependencies - e.g. in site-packages or node_nodules
E.g. say the error was about the flag single-version-externally-managed. For python projects, you can run rg single-version-externally-managed from within the .pyenv/version/3.9.1/lib/python3.9/ folder and you might find packages containing that problematic flag.