Picked up my .40 cal a few weeks ago and started having the same problem. It is NOT an issue with seating the magazine or hitting the mag release by accident. I called S&W and they sent me a new mag release which I installed but have yet to shoot and test. They seemed to be aware of the problem when they told me they would send me a new release. I have my doubts as the new release appears identical to the old one. I'm hoping they left a little more material on the release catch and I just can't see it with the naked eye. I would assume all they would need to do is re-engineer the release catch to be slightler longer where it contacts the mag. With my slide off and mag in the frame I can push the mag over to the left and make it so the smallest amount of pressure on the release will drop the mag. My guess would be the recoil of the .40 round is causing the frame to flex just enough to allow the mag to pop off the release catch.