Many mentally disturbed persons have claimed that are the mehdi. In most cases this has only caused minor problems, mainly for the person and his family. If the person has wealth or power there might be serious consequences.
This is the case with Ahmadinejad:
http://www.iranian.ws/iran_news/publish/article_10945.shtml ..."But what really has tongues wagging is the possibility that Ahmadinejad's belief in the 12th Imam's return may be linked to the supposed growing influence of a secretive society devoted to the Mahdi which was banned in the early 1980s."...
Founded in 1953 and used by the Shah of Iran to try to eradicate followers of the Bahai faith, the Hojjatieh Society is governed by the conviction that the 12th Imam's return will be hastened by the creation of chaos on earth.