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 why Othello was angry because she believed that the bond between her and Othello is unbreakable and cannot be affected by such slander.
She made the fatal mistake when Othello was about to strangle her, she did not try to escape or yell out for help. At that