This paper discusses the Monte Carlo and Molecular Dynamics methods. Both methods are, in principle, simple. However, simple does not mean risk-free. In the literature, many of the pitfalls in the field are mentioned, but usually as a footnote - and these footnotes are scattered over many papers. The present paper focuses on the `dark side' of simulation: it is one big footnote. I should stress that `dark', in this context, has no negative moral implication. It just means: under-exposed.