Writing in The Jersey Journal, Daniel Klein describes his experiences with Ancestry.com's member-contributed family trees. He describes the problem caused by novice genealogists using information from a reasonably reputable source (The US Census) and applying it to the wrong person. Now other people have accepted this erroneous information as gospel and it perpetuates over and over. You can read Daniel Klein's article at http://blog.nj.com/tracing_your_roots/2014/04/post_6.html.
I will quickly add that Klein writes only about his experiences with Ancestry.com but those of us who have been looking at online genealogy sites for years know that the problem is more widespread. It is not a problem solely on Ancestry.com, but also on all genealogy web sites that accept and republish user-contributed family trees without question or verification. Indeed, no organization can verify the information contributed by users. That would be a Herculean task.