40 Compact MP-N024X. Mag catch drop started at around 400 rounds. Sent to S&W, they replaced the mag catch, problem didn't return again until several hundred more rounds.
When I reverse the mag catch to the right side, I don't have mag drop problems.... BUT I'm not so sure how smart it is to use the gun that way when all of my other semi-autos have the mag catch button on the left.
I'm of the opinion that the placement of the mag catch AND it's relative high-sensitivity allow the mag to be dropped when the mag catch button is touched by one's thumb during recoil. Keeping one's thumb out of the way of the mag release button can prevent this, however I don't think it's a very good solution as during the intensity of a self-defense situation, one's grip is likely to be less than perfect... if not FAR from perfect... and that's one time you DON'T want your mag dropping out. So far I'm not exactly thrilled with the idea of having to watch my thumb placement or keep the mag catch button on the wrong side of the gun for me....
I have no issues of this type with any of my approximately 30 other semi-auto pistols. Otherwise, I LOVE the M&P 40 Compact!