Board Thread:Warhammer 40k General Discussion/@comment-18016240-20140707161751/@comment-4851119-20150313175817

On the topic of Traitors, do note that it's been stated somewhere or other that every Chapter except the Grey Knights (naturally) has had them. Even the loyal First Founders, as every Legion on every side of the Heresy canonically had a few guys who went to the other side.

Can't find an actual source at this time but I've seen this floated around somewhere and claimed as canon.