Iago is fortuitously assisted by the opportunity of finding Cassio and Desdemona together, giving him the idea of making Othello believe that his wife has cheated on him with his lieutenant.
Fortune also assists Iago when Desdemona misplaces her handkerchief. The personal gift from Othello to his wife is found by Emilia, who then gives it to Iago. This "evidence" helps convince Othello Desdemona's alleged guilt.
In both cases, it is luck combined with wicked plotting that assists the evil Iago.