Careful scrutiny of MCNP output is required to ensure the results obtained are the results desired. The principal sources of error are examined with recommendations on how to detect them and avoid them. There are some situations where the MCNP default physics treatments are inadequate and different options should be selected. Most MCNP physics is very robust, but other physics is more approximate such as charged particle transport, secondary particle emission, and radioactive activation and decay. Differences between MCNP table and model physics are highlighted.