Well your trigger finger of course! I have a friend and we were talking today about our glocks. He is talking about getting rid of his M17. Well yesterday he doing the pinch check (I think it is called) to make sure he has a bullet in the chamber and BOOM! Off goes the gun. So I just asked for him to show me how he did it (with an empty gun of course, I checked it first). Well he starts to do it and has his finger in on the trigger. I about fell over! Hell the momentum of the slide probably made it fire with his finger on the trigger, plus the fact he had a 3.5 trigger installed. I just couldn't believe it as this guy has many more years of experience with firearms and I look to him a lot before making most gun purchases. It just never seems to amaze me of how some people deal with their firearms. Luckily no one was hurt besides a hole in his shed. I love the glock and think it is an excellent gun all around. As with any gun though it needs to approached with respect and knowledge. Okay I'm off the soap box.