copy code at your own peril

I have come several times upon cases of scientists [I mean, real, recognised, publishing, senior scientists!] from other fields blindly copying MCMC code from a paper or website, and expecting the program to operate on their own problem… One illustration is from last week, when I read a X Validated question [from 2013] about an attempt of that kind, on a rather standard Normal posterior, but using an R code where the posterior function was not even defined. (I foolishly replied, despite having no expectation of a reply from the person asking the question.)