I just spent a couple of hours trying to uncover some additional information from Ancestry.com . Yikes! I found lots of copies of my family tree (available for free at Rootsweb.com), but with non-sensical siblings and residences and naturalization records and added wives, etc. This is a problem with any family tree, but I am surprised to see such egregious errors among the ancestry.com trees since data sources are so readily available there.
This is not unique to ancestry.com . Recently, I found some data in an online database at familysearch.org that seemed too good to be true. When I read the description of the database, it turned out that some of the information in the index was user submitted through the IGI and ancestral file collections. In other words, there was no source of information given to back up the data. I admit that I don't investigate every piece of data found in online databases, but if it is an important new find, I look up the film number (on familysearch.org) associated with the specific record to see if it was user submitted or came from a county clerk or a transcription of original records in a courthouse. We've been lulled into thinking that if it's in a database, then it is accurate/true data.
Whatever your source of data, document it. At least someone can go back to check the source someday if there is some question about accuracy.
I'm guessing that many others who research their family history treat sources like I do. I record every source. But I don't post them. My expectation was that serious (amateur) genealogists would want to contact me for my sources, thereby allowing me to make contact with them. After hundreds of hours uncovering this information, I did not want to simply give it away without at least meeting a cousin who may have information I don't. Apparently, most people prefer to anonomously copy what I've made available. But also note, serious genealogists want you to contact them.
Oh, well. Just be aware that simply because you find information in a database from a well know entity, like Ancestry.com or FamilySearch.org, does not mean that the information is accurate.
[I am not discouraging the use of these great services. I use FamilySearch often, as well as sites like FindAGrave.com and others. My point is that you should understand what the primary source of the information was and judge it's accuracy accordingly.]