Pro-tip to people at events: while you are fighting the boss, normal enemies continue to spawn, and sometimes the event even spawns more. You should kill those, too.
This weekend, there were forty to fifty people piling on a champion centaur boss (one of those fights that takes 5-10 minutes because its hit points have scaled up for forty to fifty people), and perhaps two of us were watching the outer edges for respawns. Melee respawns were no problem; they ran into a cloud of AE and fell down without anyone noticing them. Meanwhile, the Harathi Sharpshooters were forming a line. Fun math: assume 1/3 chance of getting a ranged attacker each spawn; assume death within 5 seconds if melee and respawn 15 seconds later; assume 12 spawns; how long until you have a solid line of 12 “Sharpshooters” casually standing 4 meters behind a horde of oblivious, unmoving PCs and just picking them off? Identifying where the damage is coming from can be a problem in games, and it can be loud with 40 to 50 people attacking constantly, but at some point you need to look around.