I realise that people have more trees under their user name than their own - I know I do! I have several lines with no shared matches at all - I know that illegitimacy is an issue on one- and I've been looking at people (not matches) who show these ancestors on their trees in the hope that they may have had a test to help me work something out - quite what I'm not sure
I suppose that if, according to my tree, we share a common ancestor and we're not showing as a match that there's possibly an error or another illegitimacy.... Hardly scientific but it's a start.