In decades of writing C (sometimes as a hobby, often for a lot of money) I'd guess I thought "These errors when I wrote bugs in my program are crap, somebody should fix it" maybe once per month on average. But a C compiler is very intimidating code, subtle and hard to even build from scratch let alone contribute to, so I never attempted to make such changes.

In only a few years of writing Rust (none of that paid) exactly twice I've thought "Man this compiler error diagnostic isn't very good, somebody should fix it". The first time I asked on Reddit, and I was informed that I wasn't the first to notice, the fixed diagnostic was in nightly Rust already. The second time I found the diagnostic and I just fixed it, compiled first time, wrote a new unit test, checked that passed, wrote a pull request. Landed it. Then I wrote a HN comment, a reader found a bug in my diagnostic, so I fixed the original code, and wrote a new PR which also landed. If Rust has told you that instead of 'X' when you needed a byte, you should write b'X' because just 'X' is a char not a byte - that's me, that's my small fix. [Before the fix 'X' wasn't legal here, of course, but the diagnostic wouldn't suggest what to write instead]