I just recently got into USPSA and I'm currently shooting limited with a Glock 22.
I was halfway through the stage, moving from one array of targets to another, and had performed a tactical reload. When I engaged the next target, my gun went "bang, CLICK". I saw the mag had dropped out of the magwell slightly so I tapped the mag, racked the slide, and re-engaged the target. All I got was "click". Again, the mag wasn't fully seated, so I tapped, racked, and re-engaged. Again I got a "click". So I dropped the mag and grabbed a fresh one. Everything after that went without a hitch. I actually shot the stage (30 rounds) clean.
After the stage, I examined my mag and found that I had somehow loaded 16 rounds into my 15 round mag. Not sure how I did this as the 15th round is always pretty tight going in, but I will be very conscious of this going forward.
I guess I learned 2 things: 1)Never overload your magazines, & 2)Only tap/rack once before moving on to another mag. I could have saved myself a lot of time if I had just ditched the "bad" mag sooner.
I'm sure most of you guys already know all this, but it might just help someone prevent costing themselves a lot of time and frustration on a stage.