Just because a family genealogy or a record transcription has been written down or published does not necessarily mean that it is correct. It is important as a family historian not to make assumptions about the quality of the research done by others. Everyone from professional genealogists to your own family members can make mistakes! Most printed family histories are likely to have at least a minor error or two, if not more. Books which contain transcriptions (cemetery, census, will, courthouse, etc.) may be missing vital information, may have transcription errors, or may even make invalid assumptions (e.g. stating that John is the son of William because he is the beneficiary of his will, when this relationship was not explicitly stated).
If It's On The Internet, It Must Be True!
The Internet is a valuable genealogy research tool, but Internet data, like other published sources, should be approached with skepticism. Even if the information you find seems the perfect match to your own family tree, don't take anything for granted. Even digitized records, which are generally fairly accurate, are at least one generation removed from the original. Don't get me wrong - there's plenty of great data online. The trick is to learn how to separate the good online data from the bad, by verifying and corroborating every detail for yourself. Contact the researcher, if possible, and retrace their research steps. Visit the cemetery or courthouse and see for yourself.