JOSEPH:
there were not that many false mashiachs that were reel enough that the rabanim got convinced. When it did happen, it was in a very difficult time for klal yisroel but there were still rabanim that were able to understand that it was not the real thing, just the yidden were despret to get out of the hard matziv that they were in that they did not think to much about it and just followed along with it. When mashiach comes for real, we will know its true from the rabanim that say that it is real!
The truth is that before mashich duz come, its going to be a very confusing time, and thats why we need to have a rav that we could trust that will hellp us know what to believe and what not to…