Ladislav Vagner, a legendary programming tutor at FIT CTU, is a known proponent of being extremely cautious when copying code, even your own. He gives programming proseminars where students guide him as he codes the solution to some problem, e.g. mathjax-like typesetting in C++. It is a common theme in the proseminars that a bug is introduced by copying code. Probably on purpose, like many of the other bugs that students are supposed to point out.