Parshuram,the 6th avatar of Vishnu, used to teach archery only to the people belonging to the Brahman caste. Karna,on the other hand, was unaware of his caste and went to Parshuram disguised as a brahman
Parshuram taught Karna archery till the point Parshuram himself declared Karna to be his equal.
One day Parshuram was resting on Karna’s lap, when a spider came out of nowhere and bit Karna on his thigh. The diligent student didn’t want to disturb his teacher in his sleep, so he continued to endure the excruciating pain. A few moments later, oodles of blood started pouring from Karna’s thigh and the warm blood reached Parshuram’s cheek.
Parshuram, startled, saw a stream of blood pouring out of his student’s thigh and realized that only a Kshatriya could have endured so much of pain. He rebuked Karna for having lied to him about his caste, when the fact was that Karna himself didn’t know about his caste. Parshuram, at that moment cursed him that he will forget all the knowledge he gained from his master , at the most crucial time in his life.
Karna apologized and told his teacher that he was oblivious of his caste and only knew that he was a charioteer’s son. Parshuram realizing how diligent his student was, felt guilty for giving the curse and as a reward for being his best student, gave him his personal bow, “The Vijaya Bow”, and told him that he would be unconquerable whenever he would hold that bow.