If you have a large family tree constructed, there’s a really good chance that you have something in there that simply isn’t true. It might be a birthday or a residence location or a middle name. It might also be something more significant like parentage that isn’t true, a last name pulled out of thin air, or a marriage date for a ceremony that never happened.
Most of the time, family tree errors are simply going to be mistakes that were made along the way and then repeated by family members who didn’t know any better. Most people don’t keep great records of when and where things in their life happened, so they may have accidentally reported a year of immigration incorrectly on a census, etc. Occasionally, however, people are lying about their genealogy on purpose and it isn’t until you start to dig that the lie falls apart, leaving you with nothing but questions and a not-so-favorable opinion about your ancestor.
There are many motives for fabricating genealogy. Some may be political (particularly if an association could put someone’s job, relationship, or life in danger) and others could be social, particularly if the person existed during a time when having unwed parents or an immigrant father was looked down on by society. There may also be religious motivations for fabricating genealogy, either to avoid an unwanted connection (think Eastern European jews who hid their ancestry to escape persecution) or to create connection that isn’t there. Considering these possibilities may give you more insight into what your ancestor’s life was like, why his/her motivations were what they were, and what challenges were faced daily.
As far as reconstructing a tree after you’ve discovered a falsehood goes – don’t forget that most sources are secondary sources (including family bibles, census records, and obituaries), since they are recorded accounts of one person relaying information to another. Take these accounts into consideration (even the false ones) but focus on the concrete paper trail like state records, enrollment records, military records, etc. Most likely, you’ll be able to pick up a paper trail your ancestor deliberately dropped along the way.