I was shooting in my first Texas state championship IDPA two years ago. There was a stage where we shot targets around the right side of a wall, then moved to the left side with similar number of targets around the left side. I did a RWR as I started moving from right side to left. I didn't realize that as I pulled the front mag from my mag holder, the rear one fell out on the ground. (I had someone filming behind me and you can hear a collective groan from this other shooters.). I went to slide lock with one target to go. I reached for the second mag and spent, count 'em, 13 seconds getting back my lost situational awareness, find the dropped mag, and finishing the stage. Oh, and add 3 second PE.
I had never had this happen before or since, but was using a cheap mag holder without any retention adjustment. There was a young Army Ranger in our squad that offered me one of his extra mag holders. Finished the match and ordered one with retention adjustment that night.