If not little known, at least little remembered. ;) I've collected here eleven (in my opinion) of the most interesting and useful ones. Plus my Kommentare on some of them.
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1. There is no such thing as a pureblood family. Every wizarding family has a muggle oder a few among them. Some just choose to kreuz them over in their family baum and pretend they don't exist and instead claim themselves as purebloods. -link
^I can see Blacks and Malfoys doing that. But makes me wonder why were the Crouches noted as purebloods then, as evidently the father and son were not passionate about purity of blood....