Slander and false witness will be widespread prior to the appearance of Hazrat Mahdi (as)

Verily, in the presence of the Last Hour,... there will be false testimony and concealing evidence. (Ahmad Diya ad-Din al-Kamushkhanawi, Ramuz al-Ahadith, 1/121)

The spread of false accusation is one of the portents of the Day of Reckoning. (Ash-Sharani, Mukhtasar Tazkirah al-Qurtubi, p. 450)