I went to the range today.   Next to me were 2 guys who started shooting pistols, then switched to ARs.   The first guy started shooting, and then about 2 magazines into it there was a burst of fire.  He 'looked suprised', everybody turned.   He looked at the AR, and then put it away.    I saw him in the staging area 10 minutes later on my way out.   He stopped me, asked me if I had ever seen that happen before.  I said 'See what happen?'     His story, to make it short, was that he just put the rifle together, using DPMS parts from a dealer in NC.   It was his first range trip with it.   He says about 5 rounds fired when it 'went off' in auto.   I looked at the selector switch, just the usual Safe and Fire.   I asked him specifically if he modified it, and he said no.   He also said he has never applied to legally own a fully automatic weapon.
So, can this happen, or is he BS-ing?

That's some funny shit... And to answer your question, yes it can happen... There's a few things that can go wrong that will cause a semi-auto to get happy... Usually its related to a disconnect problem or stuck firing pin...

"he just put the rifle together"
There's the cause of the trouble, right there. He probably didn't get the FCG in the lower receiver put together right, and didn't know to do a static function test before firing it.

Quoted: He probably didn't get the FCG in the lower receiver put together right.

I've installed several two stage triggers in ARs, and it's pretty hard to screw it up if the parts are in spec,

Um.  If you have the brain capacity to make chocolate milk you can assemble a lower "correctly"

Most likely he had a piece of junk stuck in the bolt carrier.

It does happen, anyone who shoots an olympic 9mm conversion knows exactly how common this can be.

Quoted: "he just put the rifle together" There's the cause of the trouble, right there. He probably didn't get the FCG in the lower receiver put together right, and didn't know to do a static function test before firing it.

worn or weak fcg springs

worn or damaged fcg

My guess would be that something went wrong.Seriously, something as minor as a weakened or broken disconnector spring, disconnect angle, or trigger could cause it. Heck, a piece of brass beneath the rear of the trigger could cause it for that much.

Happens quite often!

Quoted: Usually its related to a disconnect problem or stuck firing pin...

Agreed, I would guess he put the disco spring in the wrong way, that seems pretty common with new builds.