If it was a defective spring assembly, it's possible it might not have been noticed when the newly assembled gun was test-fired. If it was somehow damaged during assembly of the recoil spring assembly (bolted onto the guide rod), it might've slipped through until the gun was field-stripped by the new owner for its first cleaning after he/she fired it.

S&W test-fires its M&P's more than some other companies test-fire their pistols. Unless it's changed, we've always been told (in armorer classes) that each M&P is test-fired with 15 rounds (using factory test mags, not the ones that eventually get packaged with the guns).

This is compared to another major maker who (if I remember from my notes correctly) fires 2 standard pressure rounds and a single proof load (described as being 130% of standard pressure).

A long time ago we were told that in the older 2nd & 3rd gen guns, S&W randomly pulled a gun after production and fired a magazine load through it, checking for functioning and general accuracy (a random spot check, as it were).

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Ret LE Firearms inst & armorer