Ali never made any mistake. And if you think he did then you need to complain to the Prophet and Allah who made Ali your maula and aaqa in all circumstances.
Or do you think Prophet used to make mistakes too?
Imam Ali knew that Amr bin Aas being a cunning immoral person can dupe Ashari during arbitration. In letter 78 he says to Ashari:
“you should give up forming opinion about things which you have not clearly understood or visualized because many people will try their best to influence you with their insinuations.”
But he accepted him as arbitrator for the same reasons he accepted the arbitration in the first place, that is, he was forced by his khariji supporters.
http://www.al-islam.org/nahjul/35.htm
Seeing this wrong selection Amir al-mu’minin said, “Since you have not accepted my order about arbitration at least now agree that do not make Abu Musa the arbitrator. He is not a man of trust. Here is Abdullah ibn Abbas and here is Malik al-Ashtar. Select one of them.” But they did not at all listen to him and stuck to his name. Amir al-mu’minin said, “All right, do whatever you want. The day is not far when you will cut your own hands through your misdeeds.”
When Imam found out about the result of arbitration, he said:
I had given you my orders about this arbitration and put before you my hidden view, if Qasir’s (2) orders were fulfilled but you rejected it (my orders) like rough opponents and disobedient insurgents till the counsellor himself fell in doubt about his counsel and the flint (of his wit) ceased to give flame. Consequently, mine and your position became as the poet of Hawazin says:
I gave you my orders at Mun`araji'l-liwa but you did not see the good of my counsel till the noon of next day (when it was too late)