Titus' daughter Lavinia has a terrible time of it in Titus Andronicus. Given away in marriage, she is then brutally raped by Demetrius and Chiron, who also cut out her tongue and cut off her hands to ensure that she's unable to tell anyone what's happened to her. Marcus doesn't want the aged Titus to see his beloved daughter in such a terrible state, but under the circumstances he has no choice. But before he shows Titus what's happened to Lavinia he warns him that the sight will break his heart.
Marcus duly brings Lavinia before Titus, warning him with the words "This was thy daughter." His use of the past tense here is instructive. It implies very strongly that Lavinia used to be Titus's daughter, but because she's been raped and so horribly mutilated, that's no longer the case. The prevailing double standard at the time held that a woman who'd been raped was dishonored by her violation and was therefore somewhat less of a woman. Bodily integrity was considered an important part of someone's character, and Lavinia's mutilation has certainly thrown into disrepair her own integrity.
Marcus's use of the past tense is not a slip of the tongue; it is an expression of the values widely accepted by the Roman people.