I was recently reading an article in the December 2012 issue of America in WWII magazine about Charles Shulz of Peanuts fame. He said that he had a Luger that he was messing around with and almost shot a guy in the head that was across the street from him. The other individual in question was a medic and he was using the Red Cross as an aim point. Fortunately for both (more so for the medic) the distance was too great to hit his unintended target and the bullet grazed his cheek. The incident haunted Shulz for years. The article said there was a Peanuts strip where Charlie Brown was holding a pop-gun at another kid and it went "pop!" at which point Charlie Brown said, "Gee, I'm sorry. . .it went off!" I imagine that this was an all too often repeated mishap during WWII. The Luger has a very unique action which no GI would be familiar with. When there is a bullet in the chamber, a thin piece of metal protrudes slightly from the very top of the weapon and visible on the side is "GELADEN". Being unfamiliar with the pistol, it would be easy for someone (especially with the magazine removed) to think it was not loaded. My grandfather brought one back and I have shot it a few times. It shoots well, but it is easy to see how one could screw it up. The trigger doesn't take much pressure to release.
From http://www.lugerforum.com/ Just above the trigger, you can see the slot poke up when it chambers a round.
My grandfather's Luger. On the extreme left of the photo, you see the number 42. Just to the direct right of it is a 1.5mm wide slot that is about an inch long. That is what pops up and says "GELADEN". More pictures here.