The well known example of the injustice to the tribals is the story of Eklavya in the Adiparva of the Mahabharat.
Eklavya wanted to learn archery, but Dronacharya refused to teach him, regarding him as low born. Eklavya then built a statue of Dronacharya and practiced archery before the statue. He would have perhaps become a better archer than Arjun, but since Arjun was Dronacharya's favourite pupil Dronacharya told Eklavya to cut off his right thumb and give it to him as `guru dakshina' (gift to the teacher given traditionally by the student after his study is complete). In his simplicity Eklavya did what he was told.
This was a shameful act on the part of Dronacharya. He had not even taught Eklavya, so what right had he to demand `guru dakshina', and that too of the right thumb of Eklavya so that the latter may not become a better archer than his favourite pupil Arjun?