BeOS was one.

When Apple was looking for its "next generation" OS, everyone assumed that Gassé and BeOS were going to be it, but they chose Jobs and the legacy (FreeBSD-based) NextOS.

I know that "old is bad," in today's tech world, but, speaking only for myself, I'm glad they made the decision they did. BeOS was really cool, but it was too new.

I don't know if BeOS was too new conceptually. I think a lot of it at the time was that it was just incomplete. Limited or no printing. A slow and incomplete userland networking stack (later replaced by a more performant BONE stack). Incomplete and missing APIs. Single user, no real concept of multiple users.

I absolutely love BeOS - it remains the pinnacle of user-focused computing for me - but I think it would have been a tough job to base another commercial OS on it. We'll never know what could have happened, but Apple probably made the right choice in buying NeXT.