1) Graphic loading issue. I was fighting invisible enemies for well over 5 minutes. The graphic loading priority should be enemies>myself>allies>weapon effects.
2) Game crushing twice. First time I got back into another overflow server where the event was still running. Second time the event was over by the time I get back. Got zero reward.
3) Server lag. More than 7 seconds after I cast a spell, it finally casts. Not even auto attack (skill 1) was working.
4) Too crammed. These events should be spread out across multiple locations to reduce the above issues.
5) I don't mind the idea of one time events itself. But there needs to be an "instant replay" feature that allows players who missed the one time events to do it afterwards.