Our Prophet (saas) foretold 1400 years ago that news would spread claiming that Hazrat Mahdi (as) had died.

 “Narrated from Abu Jafar and Isa Bin Abdullah,  “Our Prophet (saas) decreed; “The owner of this issue, Mahdi is one of my sons. They will say about him; “HE DIED, HE PERISHED...”

(Sheikh Muhammad ibn Ibrahim Numani, al-Ghaybah al-Numani, p. 1982, al-Kafi, Volume 1, p. 340)