A Pratiloma marriage is between a lower caste man and a higher caste woman, hence have provided a story of a marriage between a kshatriya male and a brahmin female, i.e., Yayati and Devayani. There is an expanded discussion about how that marriage would be deemed bad and would lead to half-breeds.
The Mahabharata recounts (K.M. Ganguly translation, Sambhava Parva, Section 81):
Devayani answered, 'With my two thousand damsels and my waiting-maid Sarmishtha, I wait but your commands. Prosperity to thee. Be thou my friend and lord.'
"Yayati, thereupon, replied, 'Beautiful one, I do not deserve thee. Thou art the daughter of Sukra far superior to me. Thy father cannot bestow thee even on a great king.'
To this Devayani replied, 'Brahmanas had before this been united with the Kshatriyas, and Kshatriyas with Brahmanas. Thou art the son of a Rishi and thyself a Rishi. Therefore, O son of Nahusha,marry me.'
Yayati, however, replied, 'O thou of the handsomest features, the four orders have, indeed, sprung from one body. But their duties and purity are not the same, the Brahmana being truly superior to all.'
After some amount of back and forth conversation , Yayati says:
I cannot hence wed thee, O amiable one, unless thy father bestoweth thee on me.
Later Sukra, the father of Devayani agrees to the marriage. Then Yayati says:
Yayati then said, 'I solicit the boon, O Brahmana, that by so doing, the sin of begetting a half-breed might not touch me.' Sukra, however, assured him by saying, 'I shall absolve thee from the sin. Ask thou the boon that thou desirest. Fear not to wed her. I grant thee absolution.'
Then Yayati marries her. So there is a documented Pratiloma marriage which was feared by the groom as sinful, but the divine preceptor of the Asuras absolves the groom of sin in advance, and hence he agrees.
Note that he does not absolve the half-breed children (who were Yadu, the start of the Yadava race and Turvasu). It is not clear if there was any sin attached to them being a half-breed. However Yadu was cursed later and that is another story.