To add to this, a fatwa is only mandatory for two parties: the scholar who issues it, and the party who requests it. One cannot, for example, issue a fatwa demanding the deaths of all
Americans or
Jews and have it bind all Muslims. This principle regarding the scope of a fatwa is quite apart from the legal basis of the fatwa itself, which may be invalidated for various reasons. If the scholar was unaware of something, it may be disregarded by those who discover knowledge he did not have access to, or missed by accident. If his ruling contradicts the
Qu'ran and
Sunnah, the fatwa is illegal and must not be followed.
One of the issues I have is that I have heard of Muslims issuing fatwas who have not been trained in the Islamic sciences. They issue these edicts without being requested to do so, and enjoin the entire ummah to obey them. This is ridiculous, but such is the state of the ummah that Muslims are ignorant of our religion and don't worry about the authority of a source, but rather whether or not the ruling fits in with our world view. I am as guilty of this tendency as any, so don't think I simply refer to Islamists when I say this. Islam is a religion of the middle way, and some of us swing too far towards extremism and opression, and others tend towards embracing kufr and decadence.