So I'm at the range doing some magazine testing and I have some 9mm and 40 S&W magazines loaded for my G23. I finish up one of the 40's, and change magazines. The first shot is a low 'pop', and no recoil to speak of. I look at the gun thinking WTF? I drop the magazine and immediatly see the problem, 9mm in a 40. The following picture is what that brass fire formed into. The pistol was not damaged, and I finished up the range session with no other malfunctions. Leason learned? PAY ATTENTION!