The story of Parashurama and Karna is in the Mahabharata, a
significant Sanskrit epic that is very important in Hinduism.
Parashurama gave Karna a curse that his weapon, the Brahmastra
(a very powerful weapon), would fail him when he needed it
most.
This was because Karna lied about his caste to Parashurama in
order to get the knowledge of the Brahmastra after being rejected
from the school of Drona.
Karna's lie was revealed when Parashurama was sleeping and a
scorpion bit Karna- Karna didn't want to wake Parashurama, so he
kept silent, but the blood dripped down and woke him up.
Parashurama was convinced only a Kshatriya (Karna's caste) could
have borne that pain without uttering a cry, and cursed him.