[sidewndr46]

It isn't the case that a third party mistake can invalidate copyright. But if a third party does so & other groups start treating it as public domain it is on the actual rightsholder to prove they still hold the copyright.

In the case of a self published book, it's pretty obvious. In the case a movie production or otherwise, it gets difficult really fast. Throw in some corporate mergers, acquisitions, & bankruptcies and now you're looking at paying a small team of legal professionals to do research to construct a paper trail for ownership. If the work in question is valuable, obviously it gets done.

In the modern era there is a basically endless stream of video games from a 2-3 decades back where the ownership is completely unclear. The actual video game release rights might be held by one shell company, the video game source code could be held by another group (or even the original author, depending on how lazy people were), and the assets themselves might be held by another group if it was a "branded" or similar content.