One client wanted to hire me to continue developing his AutoCAD plugin written in AutoLisp (a cut down version of lisp for writing macros in AutoCAD). He had all his code in a single file which was around 63,000 lines long. (I calculated this as 504 meters of screen/paper top to bottom).
This was for a product that was in production, used commercially, and he'd been going for 15 years, adding bits as he went. There were loops hitting 300 lines, well over a hundred global variables, no consistency, and duplication like you wouldn't believe because he hadn't grasped the idea of moving code out to reusable subroutines.
I asked him what he did when his clients found a bug. The answer was "knuckle down for a few weeks".
I just couldn't believe that this was his daily work for 15 years, and he never thought to learn anything about programming other than what was immediately required to solve the problem in front of him...
Another unbelievable part of this, is that AutoCAD has an absolutely superb IDE built into it (i.e. it's fricking free!) which has features like the ability select & run code in current scope with minimal clicks which make for the fastest development environment I have ever played with, as in, you have no idea how much functionality you can churn out in a day. But he didn't like it, so he edited his code in....wait for it... WordPad! (That's a Windows built in Rich text editor where double clicking on an underscored_word only selects up to the underscore because it's not made for code, and that on its own makes it impossible to work with).
I tried to modify his code for several hours but had to stop myself because it was madness. So I told him the only way forward would be to rewrite everything from scratch after which point I could make it do anything he wanted. I reckoned it would only take 4 weeks to rebuild his 15 years of mess - partly because the IDE makes development so damn quick, and was even willing to do it on a fixed price, but he declined, which I think was utter madness.
The thing is, he had 15 high paying clients, and two other part time employees helping with other aspects of the business, and drove a brand new Audi A3, which means he probably holds a world record for highest ratio of money earned to quality of code written, at least in the CAD subcategory :-D