I am a proud owner of a Shield 40, which is my EDC. I figured with a polymer frame and Melonite finish, it would be good to go in all weather, and did not think anything of being caught in the rain a few times.
Recently, I was inspecting my pistol and found it nearly impossible to eject the magazine. Everything else functioned correctly. Once I cycled the ammo out through the ejection port, I was able to beat on it a bit. When I finally got th magazine free, I found something on the magazine release I never expected: Rust!
It turns out the magazine release for the Shield is metal, and mine appeared to have gotten a bad Melonite treatment. This is just another reason why you need to inspect your gear regularly and religiously.
For the record, S&W fixed the problem after a single email, mailing me out a new magazine release. My pistol was back up inside a week of the email being sent; kudos to the S&W Customer Support.