I already have issues when I see something but I know no-one is going to pay to fix it, so what can I do? I report it, no-one cares; I might be able to fix it but then I do this for free.

If your bugs can hurt people (financially, physically) I think you need to work a bit harder to make sure it doesn't sneak into your work. Whatever you believe helps there (unit tests, formal verification, ...); if you didn't do all you could do because you are not paid for it (your boss tells you to add features, don't waste time on things that didn't break yet for instance), what should you do? Seems like a real issue for professional coders as there are not so many options; not many companies will pay to prevent these kinds of things unless they had a big issue before already (and someone(s) got fired / sued for it already).