Only time I noticed someone doing something wrong was in a dungeon where you'd get a damaging beam from the boss, and you had to get near / behind one of the pillars around the edge. (Don't recall dungeon name, only done it once)
I saw one player not dropping the beam with the pillars and dying, so I just announced the mechanic and how to get rid of it without calling out the specific offender. Next time around, he did it correctly and we killed the boss.

If someone isn't using the right abilities, I'd have no idea, since I only play NIN, and even then, I don't know how to tell what abilities another player is using anyway.