I bought my Shield yesterday and went to the range today. Everything was ok except the extended magazine would fall out while firing almost every round. While the 6 round mag did it occasionally.
I had somebody else shoot it and it was happening to them also. We shot it one handed and stayed far away from the mag release.

I sent S&W an email but they wont be back until Monday.

What do you all think? I read that the M&P's had problems like this when they were first introduced.

That's odd. I have fired a couple thousand rounds through mine and have had no issues at all. I'd send it back. My only thought was that you would have been unintentionally hitting the mag release, but looks like you thought of that one. I think its a great pistol and S&W is pretty good about their customer service.