Ancestry.com, WorldConnect, FamilySearch, and a variety of other sites have submitted family trees. Virtually all of them contain errors. Some of them contain many errors. But don't ignore them completely. Sometimes even a very careless researcher stumbles upon something that we have overlooked. Don't take anything in the online trees without documenting it elsewhere, but consider the fact that one of them may have the clue that you need.
And some of them may raise your blood pressure when you see the errors.