According to the novel, Mago did not defy Khal Drogo, and there was no fight between them. After Drogo fell ill, Mago stole back the girl whom Daenerys saved from him. He and other Dothraki raped her, then they cut her throat, before abandoning Daenerys. When Daenerys heard what he did, she swore to punish him.
The trivia item below may give away important plot points.
Like the novels, the fate of Syrio Forel after his duel with Meryn Trant is left ambiguous. Maisie Williams later asked the show runners David Benioff and D.B. Weiss if Forel could have survived, but they replied "No, he is dead."