Lancelot was at Carbonek in one of his mad fits, subdued but unknown, at the time King Pellam’s nephew Castor was grooming for knighthood.
[S]o [Castor] desired of [Pellam] to be made knight, and so ... the king made him knight at the feast of Candlemas. And when Sir Castor was made knight, that same day he gave many gowns. And then Sir Castor sent for the fool - that was Sir Lancelot. And when he was come afore Sir Castor, he gave Sir Lancelot a robe of scarlet and all that longed unto him.
Now well groomed, Lancelot fell asleep in the garden, where Elaine of Carbonek and Brisen found and recognized him at last and cured his madness by exposing him to the Holy Grail. Under the name of Le Chevalier Mal Fet, Lancelot planned to live with Elaine in Pellam’s Joyous Isle.
Sir, said Castor ... ever meseemeth your name should be Sir Lancelot du Lake, for or now I have seen you. Sir, said Lancelot, ye are not as a gentle knight: I put case my name were Sir Lancelot, and that it list me not to discover my name, what should it grieve you here to keep my councel, and ye be not hurt thereby? but with thou well an ever it lie in my power I shall grieve you, and that I promise you truly. Then Sir Castor kneeled down and besought Sir Lancelot of mercy: For I shall never utter what ye be, while that ye be in these parts. Then Sir Lancelot pardoned him.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470