One of the people I was helping recently with a genealogy file had an interesting experience. She had copied the file from an online pedigree and had been researching the people in the file for some time. In the course of her research, she acquired a DNA study of the family she was researching and discovered that she was not related to the family. Perhaps careful research would have led her to the same conclusion, but in this case she realized that much of the research she had done was on the wrong family.
Now, I am not in a position to determine whether she was right or wrong about her relationship with that particular ancestor or to pass judgment on the merits of the DNA study, but when you copy a pedigree from an online family tree with the assumption that it is at all accurate, you are asking for trouble. Even if the file has some sources and you check some or all of the sources, you still run the risk that somewhere in the file, the wrong person was chosen.
Of course, you may make the same mistake yourself. In fact, any of us might have put a defective file online. That is a fact of the genealogist's life. There is always a possibility of error in all pedigrees. I recently had the same experience myself. I had added a line to one of my Mayflower ancestors, but because I had not kept up with the current state of the research on that particular person, I did not know that more recent evidence had shown the previous information to be inaccurate and had found a more believable spouse for the Mayflower passenger. In genealogy, all conclusions are tentative pending further research.
Just today, someone talked to me about downloading a pedigree from New.FamilySearch.org. They explained the efforts they would go through to assure that the information was correct. But I still question the advisability of incorporating other's lines wholesale, without some careful analysis of every relationship. It is always advisable to cultivate a healthy skepticism when it comes to research. I find it all too common that researchers accept other's conclusions, sometimes without question, as in, grandmother was a lifelong genealogist, she had to be right.
The work of others can be sourced and valuable. For example, going back to my Mayflower ancestor, I am not in a position to argue with the General Society of Mayflower Descendants and over a hundred years of careful and documented research. But few of our ancestors have gone through such scrutiny. In most cases, if the information in an inherited or downloaded pedigree is accurate, you should have little trouble verifying the information even if there are no cited sources. This is why we have a survey step in the research process, to provide a basis through looking at the work of others, for our own research. But incorporating an entire file without examination is not part of the research process. I have had people come to me who have never done one hour of research with files contain over 20,000 people. How are they ever going to know if what they have is accurate or not.
The quandary is whether to accept the previous information until proven false or reject everything until proven true? I think it is wise to take a little of both sides on the issues. For example, in my lines, the identities of the people back six or seven generations are not really in dispute, but once you get back that far, problems start to arise.
Fortunately, in one sense, I did not inherit a file. I had to fill out my pedigree person by person since I was the very first to computerize my particular lines. The good news is that I examined every relationship. The bad news is that I was not always accurate or correct. I have spent many years going back through my files, adding sources and correcting the entries. Meanwhile, the incomplete and inaccurate files I put online years ago come back to haunt me in the form of multiple copies by other people on the Web. Let's all live and learn.