The io.js fork from node back in 2014 or 2015 springs to mind. IIRC there were a bunch of changes/improvements that needed to be made to move node forward and Joyent were dragging their heels (a V8 upgrade might have been one of them but it's been so long I can't remember for sure). Some of the core devs were getting fed up with how long all of this was taking.

So a group of them forked off io.js from node, did the upgrade and a bunch of other improvements, and eventually all of that was folded back into core node, and everyone was happy with the final result.

But I think we could have found ourselves in a world where we'd all be using io.js rather than node had it turned out slightly differently.