The caller must be well-informed about what he is calling people to.

The Shaykh : One only issues a fatwa based on what he believes to be the Shari'ah of Allah, because the mufti represents whom?
He represents the religion of Allah and the rulings of Allah (Mighty and Majestic). For this reason, it is obligatory for the mufti to know where he is placing his foot before he places it.
He must know that this is the Shari'ah before he issues a fatwa regarding it, because he is representing the Shari'ah of Allah.
The important point is that a person must be well-informed about what he is calling people to.