I once found names of my parents and two further generations back on an online public tree. These entries were absolutely incorrect and this family had no connection whatsover with my family.
Reading further into the tree, it was one of those with zillions of names on it. There was also a facility to leave a "post-em" note, and email for the submitter.
I subsesquently emailed her, and requested that she remove my family's details from her tree, as I had proven data that they were not connected to her family. I was surprised to get a reply from her, which said something like "I finished doing my research years ago, and I am not doing it anymore. No alterations will be made to my tree". No apology - nothing!
I was pretty angry about it, so I left a post-em asking anyone viewing the tree to ignore the entries for (named the people), and that they did not belong to that tree at all. Also left my email address on it in case of bona fide researcher was looking for info about my family.
I've found another one online as well, with my brother (and the children he had with his first wife), linked in as children of he and his second wife. He and his second wife didn't even have any children together!!
Grrr!
