Why will Hazrat Mahdi (as) not be recognized when he appears?

Our beloved Prophet Muhammad (saas) has told believers of the coming of Hazrat Mahdi (as) and stated that all Muslims must join this holy individual when he does so:

O People, certain it is that Almighty Allah has forbidden you the oppressors, the hypocrites, and those who follow them, and has made Hazrat Mahdi (as), the most auspicious of the community of Muhammad, he who is in Mecca, whose name is Ahmad, son of Abdullah, your leader. Join him.(Ibn Hajar al-Haythami, Al-Qawl al-Mukhtasar fi `Alamat al-Mahdi al-Muntadhar, p. 31)

Nonetheless, the great majority of people will fail to recognize this holy personage when he first makes his appearance. Some people will even be totally mistaken, will avoid supporting him, stay away from him and engage in negative activities against him.

There are many reasons for the failure to recognize this holy person whom all Muslims have been awaiting with great excitement down the centuries. Some of these may be listed as follows in the light of the hadiths of our Prophet (saas) and of statements by great Islamic scholars