The horn effect can be triggered by small things unrelated to the job: a typo on a CV, a gap in employment, a nervous start to an interview, an accent or an unfamiliar university. Once it takes hold, assessors look for confirming evidence and discount strengths. A candidate who stumbles on the first question may be marked down on later answers that were actually strong.
Because the horn effect leads assessors to pass over good candidates, it narrows the pool, often in ways that disadvantage people from less conventional backgrounds. Defining what each criterion means, scoring questions one at a time with notes on the evidence and asking whether a negative detail is genuinely relevant to the role all limit its effect. Independent scoring by more than one assessor also helps.