Not just once, it's happened multiple times. There was also the one at the mall where the good guy was then mistakenly shot by the police, that one time 4 store patrons all pulled guns on an armed robber, those are just the ones that I can recall off the top of my head.

Actually Defensive Gun Use (hereafter DGU) estimates vary wildly, from the CDC (self) reported numbers by John Lott and Gary Kleck (500,000-3,000,000), to the more realistic estimate (based on verifiable police reports, and completely discounting the concept of defensive display) by Harvard Phillip Cook using the National Crime Victimization Survey data of 100,000 DGU/yr.

But in any case, by the lowest estimate we have, DGU still happens at least 100,000 "once"s per year, and more if we count defensive display (though that's hard to get numbers on so I leave it out of the "official" count, it does still happen, and likely more often than bullets actually get fired.)