So, I'm new to the sport. Very first match I ever shot was actually IDPA using a Walther PPQ 4" Second stage, I shot and reloaded. Shot some more and reached for my 3rd magazine to reload again to find...well...nothing. Looking at the video, you can clearly see the 3rd magazine fall out and to the ground when I grabbed my second one. I never even noticed. Had no idea where I'd lost it, so I just took the penalties for not engaging the last targets and laughed about it.
A bit later, I'm hooked and I've moved into USPSA. Now shooting my current XFive Legion in Carry Optics. It was the second stage of the day once again. I had my stage meticulously planned out. Shot my first array, reloaded (never noticing something was different), bang, bang, lock back. That day I learned that it's important to reload your magazines in between stages so that they have more than 1 round in them.