http://www.avweb.com/avwebflash/news/400-Military-Drone-Crashes-Since-2001222199-1.html
400 Military Drone Crashes Since 2001 (Over 2.5 per month)

More than 400 large U.S. military drones have crashed in major accidents around
the world since 2001, a record of calamity that exposes the potential dangers
of throwing open American skies to drone traffic, according to a yearlong
Washington Post investigation
http://www.washingtonpost.com/sf/investigative/2014/06/20/when-drones-fall-from-the-sky/?wpisrc=al_national.
Since the outbreak of the wars in Afghanistan and Iraq, military drones have
malfunctioned in myriad ways, plummeting from the sky because of mechanical
breakdowns, human error, bad weather and other reasons, according to more than
50,000 pages of accident investigation reports and other records obtained by
The Post under the Freedom of Information Act.

Defense Department officials said they are confident in the reliability of
their drones. Most of the crashes occurred in war, they emphasized, under harsh
conditions unlikely to be replicated in the United States. Military statistics
show the vast majority of flights go smoothly and that mishap rates have
steadily declined over the past decade. Officials acknowledge, however, that
drones will never be as safe as commercial jetliners. The Post’s analysis of
accident records shows that the military and drone manufacturers have yet to
overcome some fundamental safety hurdles: 1) A limited ability to detect and
avoid trouble. Cameras and high-tech sensors on a drone cannot fully replace a
pilot’s eyes and ears and nose in the cockpit. 2) Pilot error. Despite popular
perception, flying a drone is trickier than playing a video game. 3) Persistent
mechanical defects. Some drones were designed without backup safety features
and rushed to war without extensive testing. 4) Unreliable communications
links. Drones are dependent on wireless connections that can be fragile.
Records show that links were disrupted or lost in more than a quarter of the
worst crashes.