that and whenever someone claims he is the Mehdi, people take it as blasphemy and immediately start throwing stones at him, metaphorically speaking.
The Mahdi will never claim to be a Mahdi. Thats one of the signs of the true Mahdi. So yeah ..those that go around claiming Mahdiship are for sure false..and there have been several of them. Those who made claims were not only false..but they were clearly not guided. Case in point the one who tried to take over the Haram Sahreef.