A number of friendly fire incidents have been blamed on US forces being under the influence of amphetamines, which although increase alertness and reduce fatigue (while under the influence) also severely impair judgment and rationality:
http://www.globalsecurity.org/org/ne...406-iraq02.htm
http://www.guardian.co.uk/afghanista...868547,00.html
I don't believe this could merely be blamed on a communication breakdown. Even if the troops didn't know that Sgrena was on the road, there is a procedure they have to follow. They can't just go around shooting at anything that moves. I do think that this was a mistake, but there have been too many mistakes like this one. Only this time it involved people other than some Iraqis, who nobody seems to care about anyway.