I noticed this happen last night:
The fight is supposed to start off with the spawning of a number of Kwama Scribs and Workers in various different corners of the room.
After these are all killed, three bosses (...well okay, they're pretty weak) appear in the middle of the room. Killing them finishes the "group event".
BUT,
what happened was that one of the Kwama Scribs somehow seemed to have gotten knocked through the solid wall. I could see the edge of its health bar, and occasionally one of its legs waving through the wall, but it was impossible to interact with it or harm it.
After a few minutes it seemed to wander off and be no longer visible through the wall.
The trouble is, the boss kwamas of the event's final phase never showed up. And the event also did not reset, though I waited well over half an hour.
I suspect the missing kwama scrib - unkillable because it got knocked through the wall - prevented the event either continuing, completing or respawning. (Eventually I went away and completed the Endeavour to complete 1 Public Dungeon Group Event somewhere else: however, if this Group Event is not completable, it can deprive players not only of an achievement but a skill point on every character that hasn't yet completed it.)
I haven't been back to check whether it's fixed itself, but if it can happen once, it can happen again.