The sneaky thing about biases is that, for the most part, we’re unaware we hold them. To wit, “I am not prejudiced and I hate people who are.”
One way this undermines your judgement in the hiring process is what I call the “comparison bias.” If the last person you interviewed was an unmitigated disaster, the next interviewee is bound to look fantastic by comparison. And, vice versa, of course. An above-average applicant won’t look as promising when compared to the superstar you just interviewed.