Do some searching once in a while.

"Friendly-fire incidents began to garner significant public attention during the 1991 Gulf War when reporters on the battlefield witnessed allied forces accidentally firing on one another. According to the Centre for Military and Strategic Studies at the University of Calgary, recent examinations of the U.S. Civil War, the First World War and the Second World War have caused estimates of the occurrence of friendly fire to be raised to 15 to 20 per cent of all casualties from 3 to 5 per cent.

From February 2004 to February 2005, 32 attacks on British and other coalition vehicles in southern Iraq were reported as "friendly-fire" incidents. American troops were involved in a vast majority of the incidents, opening fire at night against suspected insurgents who turned out to be friendly forces. To date, at least 16 American soldiers have been killed as a result of friendly fire during operations in Iraq and Afghanistan; at least nine deaths are still under investigation."