A little knowledge
I just finished a rather good police procedural called Ice Blue. At one point, the police discover that the murdered man's daughter was actually not his. Unfortunately, the author messed it up.
The family was giving blood, and the suspect spotted how the nurse reacted when Julie, the daughter, said "Dad." The suspect casually looked at the labels on the bags and saw that the man and his wife were both Type O. The daughter couldn't have been theirs.
The problem? The daughter was AB.
Several people said that she was "undeniably" the wife's. But she couldn't have been. Two Type Os can only have O children. The real father was AB - but he and the woman couldn't have had an AB child. Julie needed to be A or B.
Sure, it's very minor. But it jarred. A little knowledge is often not enough.