I've also seen this happen when the end of the slide stop, where the spring loaded plunger rides on it, was shaped incorrectly. The angle on the end should be such that the plunger holds it down, very gently. If the fit is bad there, the slide stop can bounce up when the gun fires. I saw one gun that has been assembled improperly, and the plunger was jammed inside the tube. It wasn't engaging the slide stop at all. The slide stop was just loose. It would lock open almost every time it fired.