The first was actually a cursed sword that set into motion a series of events that would lead to the Grail quest toward the end of Arthur's reign.
2. In the earliest tales, Guinevere betrays Arthur with his nephew Mordred. Lancelot wasn't introduced until later, by a French romance author called Chretien de Troyes. There's no earlier mention of him by that name.
3. The Holy Grail is not a chalice. It's never actually described, but references to the word "grail" at the time of Sir Malory and his sources use it to mean a bowl or dish.
4. In Malory, Arthur had an illegitimate son by a noblewoman before he even laid eyes on Guinevere. According to different traditions and sources, he had many more as well.
5. Arthur had three half-sisters, not just the one. Morgause and Morgan are often conflated in modern fiction. Morgause is a woman of questionable morals, mother to Arthur's nephews. Morgan Le Fay is the most well-known, a mysterious goddess figure with roots in pre-Christian Welsh mythology. And Elaine is the least well known and largely absent.
In Geoffrey of Monmouth (the earlier document), Arthur had one full-blood sister named Anna.
What other common misconceptions have you encountered about King Arthur?