And in her eye there hath appear'd a fire, / To burn the errors that these princes hold / Against her maiden truth: In the friar's metaphor, the "errors" are not just mistakes, but heresies, violating what should be a religious belief in the truth of Hero's maidenhood. Heretics were burned at the stake in order to burn away heretical errors. princes: Don Pedro is certainly a prince (the ruler of a principality, Arragon), but the friar seems to be also referring to Don John and Claudio.