I've noticed this bug the last two times (both in the last week) I've run SToQ (NM) as a leveling roulette:

In phases 3 and 5 of the final boss, Adjudicator, he summons Mythril Verges which spawn with a "damage-taken down" buff and a shadowy pool on the ground around them. The verge immediately begins casting an aoe. After a player steps into the pool--it disappears and its borders become a force field locking players in or out and the verge loses its damage-taken down buff.

There seem to be a few bugs going on with the trap detecting players. Twice, I've had the verge just not put up the force field or lose its damage-taken down buff; it just cast its aoe while being surrounded by black goo the whole time. Repeatedly, it has done both but it has taken a while--leaving little time to kill the verge. It has happened once on NIN and once on MCH, both times I was next to the verge and hitting it.