This picture, taken last August, shows a side effect of the proliferation of UAVs on the modern battle field. A C-130 collided with an RQ-7 reconnaissance drone. Though the C-130 sustained only light damage, it was forced to make an emergency landing and lost enough fuel to make the aircraft off balance on the ground (thus the pallet tied to the wing on the right side of the picture).
Though the skies above military installations and operations have long been partitioned for use by artillery and aircraft into restricted operating zones (ROZs), drones pose a significant problem. Artillery rounds, in general, follow a predictable path to their target and thus can be given "lanes" through which friendly aircraft know not to travel. While not as predictable, manned aircraft are given zones in which to operate and also have the benefit of pilots and navigators that can ensure that collisions do not occur. In the event of multiple aircraft operating within a ROZ, through design or accident, there are also established protocols ensuring that right of way is given and the appropriate actions are taken (in general, smaller aircraft must avoid the larger ones).
While UAVs must technically avoid manned aircraft (Airspace Management, p. 11), it is understandable that in many situations the controllers simply do not have the situational awareness needed to accomplish this. The logical solution to this would be to create an object avoidance system for UAVs but this has proved to be a problem even for ground vehicles operating in two dimensions.
As unmanned craft become more common- and potentially more autonomous- on, in, under, and above modern battlefields, sites like this will likely become more common. While work is being done to prevent collisions, one must realize that this the incident above likely a best case scenario for a collision. The implications, for example, of the horse-like Big Dog trampling a human that it was supposed to be following would probably even more ugly. Similarly, a collision involving an armed Reaper drone would also be substantially less benign.