It’s easy to make an assumption about the relative you are researching. Some assumptions cause us to look in the wrong place. Some assumptions can cause us to connect people who are not really connected. And some assumptions can make think information is inconsistent when it really is not. Generally, assumptions are any statements that cannot be backed up with some sort of documentation. Incorrect assumptions can hinder our research.
And sometimes they don’t really “hinder” our research, but keep us thinking things that are not necessarily true. My Irish immigrant ancestor was a farmer after he settled in Illinois in the 1870s. I assumed that all his family back in Ireland were farmers as well. They were not and held other non-farm occupations. It was an assumption that I had made. It really did not hinder my research, but the discovery has made me realize that my ancestor was different from his relatives in a way that I had not expected.