Karna, the eldest son of Kunti and Sun God, was keen to acquire the Brahmastra mantra (the arrow which had the power to destroy the world) from Parshuram. However, Karna was a Kshatriya – from warrior class. Parashuram was a sworn enemy of the Kshatriyas. He only gave lessons to Brahmins – the priestly class.
So Karna disguised as a Brahmin and deceived Parashurama to accept him as his disciple. Karna was a devoted and able student and his hard work pleased Parshuram.
Soon Karna learned the Brahmastra mantra and became the greatest warrior on earth.
One day when Parashurama was sleeping on Karna’s lap, a bee stung Karna on the lower portion of his thigh. It was very painful and he started bleeding. However, fearing that if he moved his legs, he would awaken Parashuram, he did not move at all and continued to suffer.
When Parashuram woke up, he saw Karna bleeding.
Parashurama immediately realized that a Brahmin cannot suffer such intense pain. Only a warrior can endure so much pain.
Karna was forced to reveal his identity.
Parashuram could not digest the fact that he had taught a warrior all the knowledge that he possessed as he considered all Kshatriyas as his enemy.
He therefore cursed Karna that as he had learnt through deceit, he shall forget the skills when it is most wanted.
Thus in the Mahabharata war, Karna had to get down from his chariot to push it out as a wheel had got stuck in mud. If he had remembered his lessons he could have used other means to remove the wheel from mud without getting down and dropping his bow.
Arjuna used this opportunity to kill Karna.