Online “trees” can easily contain an error no matter how conscientious the compiler is. The number of errors can be significantly higher if the compiler is more interested in how many names they obtain or how fast they can compile information instead of how accurate it is.
Those errors can be frustrating to someone who finds them–particularly if they involve individuals the finder of the error knew personally. It can be maddening to see that someone has your grandparent with an incorrect spouse, a wrong place of birth, or a parentage that is completely erroneous. These errors can proliferate wildly if other individuals simply copy them into their tree.
The first thing a person should do is make certain that their information is correct. Sometimes this a quick process and other times it is not.
Reaching out to the compiler may get them to correct the error–or it may not. Unfortunately there is often no way to force the person to make the correction.
It is frustrating.
The best approach is to make certain your tree is as accurate as possible and that you’ve made your case as clearly and as convincingly as possible. Put that information on your tree. Consider other publication venues for the information as well.