In the play "Othello", Desdemona fell deeply in love with Othello. She was extremely loyal and faithful to the heretical general. Desdemona died at the end of the play as a result of many mistakes she made upon handling her relationship with Othello. Arguably, the first mistake that she ever made was marrying Othello, a heathen who did not match her complexion; it was a marriage that was not permitted by the society at the time. Othello's gain of Desdemona aroused the jealousy of Iago, who believed that Othello was not a proper spouse for Desdemona and wanted Desdemona himself. This first mistake Desdemona made was the fundamental cause that motivated Iago to form his conspiracy.
After the devised downfall of Cassio, Desdemona dedicated herself to speak on Cassio's behalf to reinstate him. It was not a mistake, but when Othello was persuaded by Iago's slander and questioned her about the disappearance of the handkerchief, she should have stopped her attempt on reinstating Cassio and thought about her own situation.
She simply concluded that Othello's questioning of the handkerchief was a trick to stop her from her petition by "This is a trick to put me from my suit" (3.4.101). What she should have done was to solve the misapprehension between her and Othello that was built by Iago instead of presenting her petition blindly by "I pray, talk me of Cassio", and then turning Othello away through "(I' faith,) you are to blame" (3.4.114).
Shortly after Othello walked out, Desdemona queried if the reason why Othello had behaved in this manner was some bad news from Venice. "Something, sure, of state, / Either from Venice, or some unhatched practice" (3.4.161, 162). She did not think about other possible reasons...