The last big example I can think of was XFree86 versus Xorg. XFree86 was all but stalled, with Linux vendors having to maintain huge patches themselves because the main project was so slow to accept changes. When Keith Packard, who’s personally driven X for twenty years, finally said “enough” and started organising Xorg, they expelled him.
Compare with Wayland, the new display manager to replace X in Ubuntu and Fedora. (Not a code fork, but in practical terms a developer effort fork.) The three people pushing for Wayland to replace X in Fedora are all Xorg lead developers. “Traitors”? No, people who have the actual aim in mind: making good open source display software.
Wikipedia has had any number of forks. Fred Bauder has been with Wikimedia since it was wikipedia.com — his Wikinfo fork has not led to him being regarded as a “traitor” in any way, he’s as highly respected as ever. Wikipedians have always had great interest in its forks and wished them well, including Citizendium. The community regards the forks as family, not enemies. We’re all on the same side: free educational content.
And not to mention that the project crying “treason!” at the word “fork” … started as a fork.
You can’t keep your project together with paranoia. There is no Iron Curtain around an open source project.
(There’s much wackiness around Citizendium at present. I haven’t edited there in three years, but Matt Innis has taken care to block me anyway for writing about them on RationalWiki. Gosh, that’ll sure show me! The point being, of course, internal signalling rather than anything that would affect me at all. “I will not answer any more questions and will ask the Constabulary to delete all discussions that in my view require open debate which is being suppressed here.” You can get running updates and discussion at RationalWiki. Bring your own popcorn.)