Not exactly un uncommon thing to observe when shooters are being a bit 'stressed', especially to an extent greater than just standing on a firing line and doing slow-fire target shootiung.
Once someone's attention is diverted to the task of addressing courses-of-fire in a skills oriented 'defensive' training venue, it's not uncommon to see folks sometimes lose a bit of dexterity in performing and completing tasks involving fine motor skills.
The "attention" factor can make it difficult to miss some of these sorts of things when they happen, too, and then you're behind the curve trying to identify, diagnose & resolve an unexpected problem while you're in the midst of something else.
I've watched an untold number of folks fail to properly seat a fully loaded magazine in their pistol when the slide was forward in battery in the middle of a course-of-fire when multi-tasking demands on their attention were causing their stress levels to be a bit elevated. This is right up there with them not realizing their slide has actually locked back on an empty gun and they need to reload, too.
It's much easier to get the magazine to lock in place when the slide is locked back and there's no additional compression of the magazine spring required.
When the slide is forward, though, a firm and brisk insertion technique is generally more effective than a 'simple push/shove'.
This happens with folks using rifles as well as with pistol users, too, although the magazines don't always fall free of the AR's. They generally just sit in the magazine well ... too low to permit feeding.
As far as the magazine catch issue discussed regarding some of the earlier M&P pistols?
Well, toward the end of 2007 I was told that S&W had identified the issue with some soft metal inserts in some of the earlier vendor-supplied magazine catches. Mag catches with properly hardened inserts had been going into new production guns by the Fall of 2007. S&W was either taking any affected guns in and replacing the problem mag catches under warranty, or else were sending out new ones to anyone who felt comfortable replacing them themselves.
