Such was Drona‘s affection for Arjuna that, he once demanded an upcoming an potential rival archer Ekalavya, who was a tribal boy, to cut off his right thumb as his teacher’s fee, just to ensure that, Arjuna, would remain the unvanquished one.
Drona, irked at the unknown youth, who he hasn’t coached, being as skillful as Arjuna, then played the caste card. He asked Karna, “Stop, listen, tell us, where do you come from?” “Who is your father?” “What does your family do?” “Are you a Kshatriya (the caste that the Pandavas and Kauravas belong) ?”
Duryodhana, pleased at finding somebody with whom he could potentially challenge the Pandavas, intervened. “Since when has anybody asked the origin of great men and rivers?”, he asked. “If Karna lacks any princely stature, I crown him, right now as the prince of the Anga province”
As a baby, Karna was found floating in the river by Adiratha, a low caste charioteer. He grew up as Adhiratha‘s son. Thus nobody knew to which caste did Karna originally belong to. For all practical purposes he was the charioteer’s son.