You get to see quite a few things when you are in a training class often enough. Firearms malfunctioning is one of them, but you can also witness magazines malfunctioning.
Here is a SIG P226 magazine that fell to the ground. As you can see, the first round nose dived in to the magazine. This is not an isolated incident. In some extreme cases, the first round will actually turn around and bullet will end up facing back of the magazine. This happens since the magazine spring does not hold up the remaining ammunitions, and compress when hitting the ground, making enough gap for the top ammunition to tumble.
Another problem witnessed is follower not pushing up the ammunition. Inside the M&P magazine, there are two ammunitions that refused to go up. This is something I have not heard happening but here it is.
Both incidents are not frequent occurences, but it does happen from time to time. This is something that is not going to happen on a square range where dropping a magazine doesn't happen that often.
It doesn't take away from the fact that both SIG P226 and S&W M&P are overall great guns. There are no guns that are reliable 100% of times. It seems like the best preparation is to expect such malfunctions to occur and be ready to deal with it.