Countless examples either way. It can lead to a "nodejs/iojs" situation where the fork happened because of disagreement, and the upstream project realizes their mistake and eventually integrates the fork into mainline development repository. That probably would count as a success for everyone.
Or it goes the way of webkit/blink, where both forks are successful in their own way, although the project they forked from (khtml), probably is considered less successful.
Either way, that it's even possible to fork and continue working on both I'd consider a success in it's own way. It's the beauty of FOSS.
Maybe the best fork win, and also the base it was forked from.