Patrise of Ireland
One of the guests at a small dinner party of Guenevere's, Patrise was accidentally poisoned and killed by eating an apple meant for Gawaine, who loved fruit and had an enemy in Sir Pinel. At a dinner Guinevere held for a couple of knights of the Round Table Pinel succeeded to poison an apple in the fruitbasket, sure that Gawaine would take it. Patrise took it instead, bit it, chewed, ate, becomes swollen and cracked.
The rest of the company were convinced the queen were behind it all, and when Patrise's cousin Mador de la Porte, accused her in front of the King he had nothing else to do than to sentence her to death, unless a knight could prove her innocence in a single-combat with Mador.
No one voluntered though, because no true knight don't fight for a unjust cause, and everyone thought of her as guilty. In the last moment Lancelot showed up - it happened to be one of the many times when the queen, out of jealousy, had driven him from the court - and fought Mador de la Porte, who were defeated and then took back his accusation.