Punishment for eating/mingling with lower castes according to Hinduism

 Parashar Smriti 4.8 - 12

Having unwillingly eaten or dealt with a person of low caste, one would remain equally degraded for a fortnight, month, two months, six months or a year.

A fast of 3 days should be observed for the first fortnight, a Kriccha Vrat should be performed for the second, a Kriccha Santapana Vrat should be performed for the third, a Dasharatra Vrat for the fourth, a Paraka Vrat for the fifth, a Chandrayana Vrat for the sixth, two Chandrayana Vrats for the seventh and a Kriccha Vrat covering a period of six months for the eighth fortnight, of such defiling contact.

The penitent should make as many gifts of gold to Brahmins as the number of fortnights he might have lived in the company of a person of low caste person.