Yes - that should not happen. Call Walther USA/S&W tomorrow. However - as you may be unfamiliar with the mag release - are U sure you weren't hitting the mag release yourself?

Does it happen with all your mags or just the one? Is it the first generation or newer. Shipwreck is right, on the newer P99's the mag release is longer and it is possible the flip up from recoil is causing the trigger finger to hit the mag release. Recoil is pretty good with the .40, Not a normal occurance, but it has been known to happen.

TommyRox, I shot rounds 200 through 300 through my P99AS yesterday and the clips were doing exactly what you described. This problem just started yesterday and progressively got worse. It would do it at least once for every clip. It's not a faulty clip since it was doing it with both of mine. Shipwreck's question was my 1st thought but quickly dismissed considering I would have been doing that from day one and you have to pull the release much further to operate than a finger bump can do from inside the trigger guard. My best guess is that the mag catch spring is not strong enough to withstand the recoil and is rapidly weakening. We should do as advised and contact Walther. I will post what I find out. Please do the same so we can compare notes and help each other resolve this issue.
By the way...other than that, I LOVE the gun!

Just noticed the dates. Haha

I purchased a P99 in November 2011. Shot a couple hundred rounds and then started having the same problems you are having. I sent it to Walther USA and foundnout that the frame was cracked and that was the cause of the problem. Walther is going to replace the gun with a new one. The new one is on order from Germany. I expect to receive it in the next week or so. Hopefully the new one wont have the same defect but hearing that your problem is virtually identical to the problem I had it appears that the problem may be with an entire production run.

I just posted a reply below and when the same problem started I carefully checked everything I was doing to make sure that I was not releasing the mag. I also used each of my 4 mags and the problem got worse and I was nowhere near the mag release on eithernside of the trigger guard. That is when I called Walther. They sent a shipping label and I sent it to them in January. Found out the problem (cracked frame) about 2 weeks later. Their head gunsmith had to sign off on replacing the gun with a new on and I was then called by someone in service. My new gun is on order.

No plastic!!!!!!!!!!