I know it when I see it…
Barring concrete examples, I'd say pathological is anything no sane programmer should seriously be doing. An attacker might be trying those things in order to find exploits somewhere (compiler, runtime, …). A programmer might be doing them for sports or fun, but knowing that this is nothing to do when delivering code for production use. A novice might use pathological programming styles due to a misunderstanding of how the language is designed.
Compare this to pathological inputs. Some algorithms perform quite well in general, except in some situations which are usually irrelevant in real applications but which break the worst case behavior. Depending on your point of view, pathological inputs may either be rare enough to completely disregard them, or particularly problematic to focus on them. I'd assume the same to be true for pathological programming.