I can think of a couple examples off the top of my head --- TeX and template metaprogramming.  Although I think Knuth protests too much when he says that he tried not to make TeX Turing complete.  At any rate, in both cases, Turing-completeness should be considered a hazard.  There should be a warning label on TeX, particularly:<p>WARNING: TeX is Turing complete.  Proceed with caution!