Online family trees are notorious for being unsupported by source citations and other failings. Some genealogists refuse to put their family trees online likely because they don't want to mingle with the unwashed masses but sometimes for reasons of privacy and incomplete research. Cooperative and collaborative family trees receive the greatest number of complaints. The existence of millions upon millions of online family trees and increased DNA testing will eventually confirm relationships for many people without a significant individual effort at documentation.
I decided I would categorize the biggest problems with online family trees. Of course, my list is purely subjective but I have examined thousands of family trees over the years and I am pretty well aware of the more egregious failings. I am not going to try to put my list in any kind of ranking or order because the problems are all equally challenging. Here I go with the list.
1. Having a DNA test published online without a supporting family tree.
I have thousands of DNA matches on two different websites but less than 2 out of 10 of those DNA test results are accompanied by a significant family tree. In fact, most of the family trees listed have fewer than 50 entries. This means that absent a way to contact the people, I cannot tell how I am related to them. Because I have an extensive family tree and know many of the surnames of my living ancestors, I can often guess about the connection. Those people who thought the increased interest in DNA testing would fuel a concurrent increase in interest in genealogical research have yet to see that happen. Interestingly, websites such as Ancestry.com and MyHeritage.com provide me with guesses as to the common ancestor even with insufficient information.
The lack of family tree information is surprising since the DNA matches do make connections to people, such as me, with extensive family trees.
2. Missing, inaccurate, or incomplete place names for events in the family trees.
I have written about this issue many times in the past. Without accurate information about the exact location of an event in an ancestor's life, there is no real way to be assured that the right person has been entered into the family tree. I see an abundance of places that say the person was born in England or New York or some other general location. These entries are very often wrong because someone has entered an individual with the same name but not verified that the person is the same person. Accurate place entries are indispensable.
3. Incomplete, missing, or inaccurate names.
Although not nearly as serious as inaccurate or missing locations, names are important. But this importance works both ways. Having the wrong name is misleading but even the right name might not help all that much. Most names are fairly common. Using names to search and identify only works when the rest of the identifying information is consistent. But adding a female's married name in European derived countries except those that have Spanish or Latin naming patterns is misleading. Other problem areas include all of the countries that have extensive patronymics.
4. Extending pedigrees based on supposed family traditions or beliefs.
According to my own family traditions, I was supposed to be related to some famous people. Some of those traditions turned out to be myths. This is not uncommon but traditions about connections to royalty and Native Americans are more common. Granted, some of these extended pedigrees are based on historical references but I commonly find that there are few or no sources supporting the claimed connection to the first royal or noble family member.
5. Including children in a family who are not verified members of the family.
This is an extremely common problem and one of the reasons why I could not rank these issues. Depending on your personal experience, you could easily rank this as the #1 problem. The main cause of this problem is adding a person to a family without verifying the location of the family and finding a source that confirms that the child was an actual member of that specific family. This issue is particularly common in countries such as Denmark, Sweden, and Norway where there are so many people with the same names. The solution to this problem (and almost all of the others) is careful, knowledgeable research.
6. Merging individuals without enough information to determine if they are the same individual.
This is another same name = same person error. This problem is also confined to collaborative family trees such as the FamilySearch.org Family Tree that actually have a way to merge duplicates. Although, you can find and merge duplicates on websites that host your own personal family tree such as Ancestry.com. Merging is a fairly complex procedure and is only warranted when the information is sufficient to adequately establish that the possible duplicate is really the same person.
7. Copying information from an old GEDCOM file or from Personal Ancestral File without systematically verifying all of the entries.
This is really two problems in one. The first problem is assuming that the GEDCOM or PAF file you inherited is correct and verified. The second is the problem as stated above; adding that information to a collaborative family tree or using the information as the basis for your own family tree. Either way, you are probably inheriting a lot of inaccurate information even if your donating ancestor was a "professional" genealogist. We aren't infallible and none of our ancestors were infallible. Just don't do it.
8. Violating one of the current Twelve Rules of Genealogy.
9. Using a fan chart to determine what work "needs to be done" on your family tree.
When someone asks for help with their family tree and begins by pulling up a fan chart and pointing to a blank space, I almost always determine that they know little or nothing about all the people connecting them to this elusive ancestor. I can say a lot more about fan charts such as the fact that they ignore children and other relatives, but so do traditional pedigree charts to some extent. It is just too easy to ignore the accuracy of a pedigree line using a fan chart.
10. Failing to research or record the source where information was obtained for events in an ancestor's life.
Documenting the source of your information is absolutely essential for an accurate family tree. A source is the only way that you or anyone else can tell if what is recorded has some basis in reality. A family tree without valid sources is pure fantasy. But remember, your personal information is a source. But if you do some research, you may find you do not know your own birthdate and your parents are not really your parents. Never assume anything.
You may have some more of your own pet peeves and you are welcome to add them by comments.