Bias is something you've got to look out for, but it can sometimes be difficult to see if you're not reading multiple sources. I remember one author writing about the execution of the Pappenheimers in Munich, Bavaria 1600. The entire family, all five of them were accused of witchcraft with all but the the youngest, a boy of 10, was sentenced to death while he was spared on account of his age. He was forced to watch the executions as they thought it'd do him some good. The parents and their two eldest children were executed in a manner so brutal it would be a violation of the board's rules to post it here. The author was trying to go into detail about how much more brutal executions were for women, but she completely ignored the executions of the male Pappenheimers. i.e. I felt she was cherry picking her source.
In graduate school, I wrote about the lynching of John Carter in Little Rock in 1927 which was the last big lynching event in the state. The paper was completely finished three weeks before I had to turn it in, but on a final review, I noticed I had not included the perspective of any African Americans, so I spent the next two weeks fixing that error. But it's something I could have easily missed. Sometimes bias is unintentional but other times it's as if the historian has an axe to grind.