In some parts of the world, particularly for people who descend from ancestors from Western Europe, genealogy is not a particularly new innovation or pursuit. In my 43 years of doing genealogical research, I spent the first 15 or so years copying genealogical information from my ancestors. This information was found in books and a huge number of family group sheets located in the Salt Lake City Family History Library. The information that I found was sometimes incomplete and inaccurate. For example, I found this book in my father's personal library.
This book contained a short section about John Tanner's ancestors going back to one William Tanner from the Rhode Island colony in the 1700s. Because it appeared that all the information was available in this book, it seemed that all I needed to do was copy the information out into my own genealogy computer program. (My use of computers for doing genealogy started at about the same time.) The problem was that the information about "William Tanner" was both partial and inaccurate. Many, many years later, after beginning actual research, learning a lot more about genealogical research, and looking at original source records from Rhode Island, I boiled down most of the inaccuracies to the simple fact that no one had yet identified William Tanner and had certainly not traced his origin to England.
Now, what is happening with the online family trees and William Tanner? A huge number of William Tanner's supposed descendants are still copying the inaccurate and incomplete information out of the John Tanner book cited above. The reality is that I have found as many as ten men living in the Rhode Island Colony during the same late 1600s to 1700s with the same name: William Tanner. The results are evident from the constant ignorant changes and additions being made to the FamilySearch.org Family Tree entry for an entry many consider to be the person from the book. Here is a screenshot of the entries you can find with people who may or may not be THE William Tanner who is the ancestor of John Tanner from the book.