The genealogical community is plagued by an epidemic of false noble medieval ancestries of immigrants to colonial America. Some of these false lineages have been disproven for decades, but the bogus information is still very much "out there" on the internet. Some of them are based on groundless speculation that has been repeated as "fact" at places like ancestry.com, familysearch.org, the World Family Tree, and various collaborative family trees including wikitree.
This is an issue of special concern to wikitree projects which deal with immigrant ancestors to colonial America, as well as the Magna Carta project of which I am a co-leader -- there are far too many bogus pedigrees linking immigrant ancestors to Maga Carta barons. I think this is also an issue of general concern to wikitree, because the presence of so much garbage ancestry damages wikitree's credibility.
How big is this problem? When I first came to wikitree, I found that about one third of my immigrant ancestors to colonial Plymouth Colony had false ancestries, usually going back to medieval noble families. Since then I have found repeated evidence that this is part of a widespread general pattern. And repeatedly, as I have detached false lineages, I have encountered resistance from well-meaning genealogists who are strongly attached to their imaginary ancestors! I try at all times to be tactful, but sometimes people just don't want to discuss evidence or the lack of it.
For whatever it's worth, I've been stumbling across and debunking false royal ancestries (and getting over repeated disappointment) ever since I was a teenager.
So... how to distinguish genuine medieval lineages from false ones? For immigrants to colonial America, the starting point is the list of recognized proven "gateway ancestors." The Magna Carta project has a list of these gateway ancestors here: http://www.wikitree.com/wiki/Space:Gateway_Ancestors_-_Magna_Carta_Project
If you see a colonial immigrant with an illustrious noble ancestry who is NOT on this list of gateway ancestors, then the ancestry is almost certainly false. If it is not false, then that immigrant should be added to the list of gateway ancestors!
Further tips for sniffing out false medieval lineages:
1. Check the sources on the wikitree profile. If the only source is ancestry.com, be suspicious.
2. Do a google search using the names of the parents of the immigrant ancestor. If the only results are links to un-sourced personal family tree sites, this is cause for great suspicion. Proven lineages almost always have at least one high-quality webpage with good documentation.
3. Check the dates! Many of these false ancestries are woefully incompetent, showing a parent born ten years before the child (for example), or a parent born a hundred years before the child. Sometimes the immigrant ancestor has proven grandparents, but it is an earlier generation that has a false link.
4. Look for sudden jumps from one county in England to a different non-adjacent county. This often shows that somebody arbitrarily linked two families, based on nothing more than a common surname.
Now, for some examples of bogus medieval pedigrees:
--Robert White (c. 1558 - 1617) of Shalford, Essex at http://www.wikitree.com/index.php?title=White-255&public=1 -- all of his children came to America and I've seen the claim that he has more proven American descendants than anybody else. He has a widely publicized imaginary ancestry, and his wikitree profile gives an example of how to handle false parentage -- detach the "parents" and add links to them in the text.
--Henry Adams (1583-1646)) of Braintree, Massachusetts, ancestor of two American Presidents, at http://www.wikitree.com/wiki/Adams-277 -- in this case, his royal ancestry was disproven a while ago (as discussed on his profile), but the bogus ancestry got merged back in, until it was recently pruned. Lesson here: these false lineages are like weeds, they keep growing back.
--Here's one from Virginia, that still has its false imaginary ancestry (conclusively disproven by DNA evidence) attached at wikitree: Ralph Shelton (1685-1733) at http://www.wikitree.com/wiki/Shelton-52
And finally, here's a newly-discovered set of possibly bogus acestral lines needing further research, the family tree of Isaac Howland, grandson of Mayflower pilgrim John Howland: http://www.wikitree.com/genealogy/Howland-Family-Tree-97
This tree shows Isaac's maternal grandmother Mary (Atwood) Lee descending, up the Atwood tree a bit, from Margaret Grenville. If this is correct, then Mary is a new gateway ancestor, because Margaret Grenvill has proven Magna Carta ancestors. More research needed! A preliminary google search gives this suggestive website: http://www.genealogy.com/users/l/e/e/Janet-Lee/FILE/0006text.txt
Isaac Howland's paternal grandmother, Mayflower passenger Elizabeth (Tilley) Howland, was the daughter of Joan (Hurst) Tilley. Wikitree currently shows Joan's paternal grandmother as Agnes (Dalton) Hurst, which appears to be the worst sort of absolutely groundless internet fabrication. A good summary of what is actually known about the Hurst family, mentioning reputable published sources, is here: http://www.boydhouse.com/michelle/gorham/henryhurst.html