How odd, I've had nothing but seemingly clean games with randoms since launch all the way through to Suicide difficulty, then I've run into two within then hour tonight.
First guy wasn't actually doing anything mental, I only realised something was up 3/4 the way through a mission when we got swarmed from behind and he went crazy with the grenade launcher firing off a good 20+ shots without reload. I kept an eye on him afterwards and he never reloaded or supplied up from what I saw so he might have had infinite ammo.
Second one I'm not even 100% who it was, maybe multiple players, but it was a little more worrying. We had one of those random groups where people are just running off on their own trying to do side missions by themselves. One guy ran ahead, got killed, then me and another guy went to his spot to finish the objective and picked up his dropped materials. The person who picked them up sent a message in chat saying something like WTF? Glitch? loads of samples here? He then left a few moments later leaving me on my own in that area.
I made my way back over the map to where the other two now were, arriving just as one of them died, I picked up his dropped materials and picked up something like 5000 of every type in one go. I took the same approach as the other guy and just left straight away, didn't want any part of whatever was going on there.