How come Benei Israel Tau (made mistake) on the 9 B'Av, it happend after they got the Mitzvah of Kiddush Hachodeh, so whatever THEY decided 9 B'Av is, this is 9 B'Av.
Then why they thought that they may made mistake?
I am unsure as to what you are referring to. Perhaps you mean to ask about the Gemara on 29a that Tamuz of that year was extended (i.e. made to be of 30 days instead of 29 in order that the spies' return would fall on the ninth of Av. However, this was not a mistake, rather the Beis Din of the time decided that it was necessary to extend the month, presumably because there were no witnesses who had seen the new moon on the twenty-ninth.