See it every class. People stuff as many rounds as they can into a mag then stuff one more. Middle of a drill, they reload, press the trigger and while the gun fires, the mag decides to exit stage bottom. Usually we see this with double stack standard capacity mags. This past Saturday we had someone with an EMP that if the mags were filled to capacity they would not seat unless you sat on the mag with the gun on the bench underneath it. Loading one round down did the trick. Before discovering that, they had basically beat their palm senseless trying to seat mags during class.
So, if your mad holds 11-20 rounds, load one down, if it holds 21+ load two rounds down. This will save you much grief and embarrassment and who knows, it could even save your life some day.
Next minor gripe, know your gun. practice with it enough to know if it has quirks. Don't buy something new and expect it and you to perform flawlessly together right out of the box.