Runaway Drone Shot Out of Sky

An MQ-9 Reaper drone's fail-safe programming failed and it rushed through Afghanistan's airspace on Sunday. The US Air Force simply responded by sending a manned plane to shoot it down. Turns out this isn't all that uncommon.

Apparently the pilots of an unmanned, but armed, MQ-9 Reaper drone lost control of the UAV a few days ago. The resulting manned mission to take it down was mounted almost immediately and ended successfully before the drone reached the edges of Afghanistan's airspace.

I wonder what would've happened if the Reaper hadn't been shot down. It is typically used for "hunter-killer" missions and targets enemies on the ground. Would it continue a mission like that without operator input?

Unmanned Aerial Vehicles like the Reaper are supposed to be programmed to return to their home bases in case they lose contact with their pilots. There's no explanation as to why the fail-safe didn't kick in for this particular Reaper. Nor is this incident the first time that a UAV has been shot down by intentional friendly fire. What's going on and when can I start to panic? [Aviation Weekly via Popular Science]