If "done properly" implies "works ok", then it's circular reasoning. I'm not sure your example is a counterexample either. Implementing part of a fairly simple

If "done properly" implies "works ok", then it's circular reasoning. I'm not sure your example is a counterexample either. Implementing part of a fairly simple runtime (like Lua's) using longjmp is an example of a good use of longjmp, in my opinion.