Oh I'm totally an armchair lawyer, so my ruminations were not grounded in laws or legal precedence :-) I do have some background on the patent side of things, where independent reinvention is also not a defence for infringement, but not so much in copyright, so this was educational.

However, has there been any case where the infringment was not only unintentional, but also unexpected?

That is, if you look at cases of uintentional infringement, these are typically cases where some the act of reproduction of content was intentional, but there was a lack of awareness or confusion about the copyright protections of that content. (This paper was useful for background: https://www.law.uci.edu/faculty/full-time/reese/reese_innoce...)

But I could not find a case where the act of copying itself was non-intentional.

In this case, looking at how LLM training works and what LLMs do, it is surprising that it could reproduce the training content verbatim. The fact that it reproduced those outputs is undeniable, but how does existing law and jurisprudence apply to an unprecedented case like this where the reproduction was through some magic black box that nobody can decipher?