If in fact someone put a live round in a box of blanks or dummies, then the armorer was indeed incompetent not to recognize the difference and check. Since there are multiple accounts of the crew "plinking," it's unlikely that the armorer didn't know that. People don't plink with blanks.
Very surprising that the armorer somehow chose this one live round (I believe more will be found and in fact believe the Sheriff mentioned that they found live ammo on the set - I'll have to check again). She chose one live round and put it in the cylinder and spun the cylinder so that round was the one that would be shot during this supposed rehearsal (where...gun could have been totally unloaded or should have been using dummies). Why they would need dummies for rehearsal mystifies me still.
First, we heard there were only 3 rounds total in the gun. Today, on two different news stations, it's been reporting that there were 5 rounds in the gun, and one reporter said 6. Now, Hall (the AD) is saying that he did check the gun with HGR and that there were 5. I think he says he should have checked more closely (he obviously assumed they were...what? blanks or dummies? why won't anyone say?...they're talking so much, might as well give the details).
Blanks kill people too. And the chances are greater than one in a trillion (as people keep saying - I suppose they're saying that out of all prop gun shots ever fired - and apparently there's this ginormous amount - in the trillions - of blank rounds shot...only a handful of people have been killed. There at least two before this shooting - so that makes 3.
I'd calculate it a bit different. How many people are on the planet? About 8 billion. How many have died from prop gun injuries since, say, 1980? 3 that I know of. So it's more like a 1 in 2-3 billion chance (rare...but...certainly not 1 in a trillion...IMO)
Anyway, it's so odd that if there was just this one live round on set...it ended up being the one behind the hammer during this rehearsal.