Got my fresh Guardian to 55 at the weekend (9 calendar days from creation screen to 55 ding) and the first 50 HM I queue for on Sunday morning was Kaon Under Siege. Sage healer (55), commando DPS (53), Sentinel DPS (50), and me, Guardian 50 (wearing about 4 pieces of Black Hole I'd picked up in normal-mode D7 and False Emperor, and a smattering of BoE blue or purple guardian gear that has been sitting in various cargo holds).
After a few fights, I realise our DPS was a little low, and looking around, I see that the "DPS Commando" is actually using Combat Support Cell, and is basically using Hammer Shot, Full Auto, and Explosive Round. Oh, and he's got Trauma Probe on himself. In other words, he seems to be in full Heal spec, but he must have queued as a DPS ... I ask him why he's doing that, but I get no response, he doesn't even change the cell...
The other 3 of us, even me, are adequately geared and know enough of the tactics to cope with this guy, even though he keeps blundering into packs of trash, and standing next to the bloated plaguebearers as they're about to explode -- he dies a lot, but hey, it's all XP for me since he doesn't actually manage to wipe us, until we get to the Bonus boss (the Droid) -- who actually enrages at 20% because this guy is still using the same attacks he got prior to level 10. I've reached the end of my patience with him, and I'm about to vote-kick him when the sage, who's been very laid back so far, votes kick on "Mr DPS Commando", I think the reason was "clueless"
We got a gunslinger DPS as replacement, and needless to say, we easily killed the bonus as well as the last trio.