Applying @pytest.mark.slwo instead of @pytest.mark.slow is not an error. pytest emits PytestUnknownMarkWarning: Unknown pytest.mark.slwo - is this a typo? You can register custom marks to avoid this warning and continues, so pytest -m slow runs zero tests and reports success.
Register every custom marker in the configuration under markers, one per line as name: description. Once registered, the warning distinguishes real markers from typos, and pytest --markers lists them with their descriptions.
Turn the warning into a failure with filterwarnings = error::pytest.PytestUnknownMarkWarning in the ini file, or use --strict-markers on the command line, which makes an unregistered marker a collection error. Adding --strict-markers to addopts is the version that protects everyone rather than whoever remembers the flag. The same reasoning applies to -k expressions, which match against test names and silently select nothing when misspelled, so check the collected count rather than trusting a green run.