Parashuram Curses Karn

Karn wished to train under Lord Parashuram. But the guru trained only Brahmins; he hated Kshatriyas with a vengeance as they killed his father, Rishi Jamadagni. So Karn, a Kshatriya, disguised himself as a Brahmin to become Parashuram's student. He eventually learned all the mantras from the revered and feared teacher. 

One day, when Parashuram was asleep resting on Karn's lap, Indra, the king of the gods, transformed into a bee and stung Karn's thigh. Karn began to bleed but didn't move a muscle because he didn't want to awaken his teacher. But when Parashuram's clothes became soaked with blood, he awoke with a start. Since a Brahmin could not possibly withstand such intense pain, Parashuram realized that Karn was a Kshatriya in disguise. Furious, he cursed Karn:

"You will forget my teachings when you need them the most."