Likewise, I've found many bugs using LLMs in the small software libraries I hand wrote and considered complete and correct - even though it was my sole focus at the time and I thought I had perfected it

Same. Frontier LLM code review proved to be a very humbling experience for me.

I have had the same experience.

I’m still in the process of revisiting and refining the many hand-coded dependencies that I’ve created, over the years.

Most of the issues found, were corner cases, that would likely never be encountered, but they are issues, nonetheless.