This has happened before. Just not sure how many times. On an Asian airline, possibly Japan airlines as I recall, as the details were presented on an episode of "Mayday" and seemed hauntingly similar to this Germanwing crash.
When I heard the initial details of this recent Germanwings AB 320 going down in the Alps I thought to myself ....oh no not another sabotage? I just couldn't believe it to be true, that it would happened again but as it has turned out the similarity I noticed looks to be proven now.
The details on the Mayday reenactment were that the pilot had left the cockpit to apparently go for a washroom break as I recall but then he turned off some responding equipment or indicating alarms while he was away at the rear of the plane. He then he made his way back to the cockpit and then invited the co-pilot to leave to take a break etc. Once the co-pilot left the cockpit the pilot locked the door, turned off some additional inicating equipment and flew the plane straight down into a 600 mph dive into a swamp. Apparently the pilot was an experienced fighter pilot in his past - they said that steep dive would be very hard to do and hold for any pilot. The wreckage was apparently found up to 20' below the mud in the swamp.
The pilot was apparently found to be having depression prior to flight and had just lost a million bucks on stock investments
As SF said, I also feel there should be some sort of software/hardware "lock" built into aircraft such that the aircraft won't allow itself to crash when it detects a drastic manual maneuver of sabotage . Also if the pilot in the Germanwings incident had been able to access the cockpit with the code ( or accessed some sort of an emergency control override outside of the cockpit) that may have saved the situation.