I talked with my dad on the phone yesterday and he told me of an "experience" he had recently shooting CCI Blazer in his Smith 686+.  I forgot the weight, but they were magnum loads.  At the local outdoor range, he was just about finished, and noticed he had 3 rounds left in the box of 50.  He put them in the cylinder, shut it, took aim, and fired.  

What happened next was a very loud explosion.  Apparently the aluminum case separated and as it did, gas followed the path of least resistance, which is the cut-out in the rear of the cylinder for the ejector.  The round immediately left of the fired round went off as the gas burned through its aluminum case, sending the bullet into the frame, and fragments, powder, and you name it , eveywhere.  Luckily, it did not set off the 3rd round.  The frame of the gun has several scars, but nothing serious happened to it, but more thankfully, nothing serious happened to my dad.  His left hand was "peppered," but that's all. Nothing hit him in the face, eyes, etc, plus he uses shooting glasses.

He talked with CCI on the phone, and is not going to have the gun repaired by them, since any "damage" was slight and cosmetic.

Draw your own conclusions, but this reminds me why shooting safety is so important.  Things happen that you don't expect, and even though he didn't get anything in the eyes/face, he's really glad to have had glasses on.

Glad he wasn't hurt seriously. I have been shooting Blazer ammo since the late 80s and have never had a problem. I shoot more Blazer ammo than all others combined, including handloads. It just goes to show you that a crappy round can end up in any box of ammo no matter the manufacturer.

I have a hard time understanding how your Dad had a "flashover" discharge with metallic ammo, even aluminum cased. I can understand the "kaboom", the rim probably separated from the case. Since the adjoining round has only a small portion of the rim showing I can't see how it could have flashburned thru it and ignited the powder. One would think that the adjoining  bullet would do some serious damage to the gun and at least knock off the timing. I would caution your Dad to have it looked at by a gunsmith to check the timing and to make sure damage that cannot be readily seen has occured. Did a bullet get stuck in the bore?

My dad works at a firearms retailer with a gunsmith, so it's been checked out. I'll ask him next time we talk if the round fired left the bore or was stuck. He told me the primer on the second round has no dimple in it, and you can see where the flame burned through the 2nd case.  I am nowhere nearby, so I can't look at it myself.

I think it's safe to say that this was an isolated incident.  It could probably have happened with any ammo, as I'm sure other people have had similar experiences with the occasional "oddball" round that left the factory.