So yesterday while doing a run of Qarn, one of my companions was killed inside one of the barrier walls that come up when you have to defeat the key before it bursts.

After the remaining three of us killed the boss, our healer attempted to rez him, but found that our Summoner was untargetable. I walked over to where he had died, and found that there was still an invisible wall there precisely around where a key spawns. The Summoner returned to the beginning of the dungeon and ran back, but the wall still remained.

It's not a serious bug, but something to be noted.