Not considered "in combat" if too many players are attacking the same target.

Steps:
1. Simply start attacking something that's already being attacked by several players (S rank hunts and notorious monsters in Eureka are the chief offenders)
2. If the number of players present is high enough, you won't register on the monster's enmity table, and by extension, will not be considered to be in combat, which means you will/won't be able to use certain skills despite technically being in combat.

This is an issue for MCH particularly, because they rely on being able to use Flamethrower at the start of a fight to overheat their weapon and unleash their burst damage. The problem is, if you aren't considered to be in combat, Flamethrower won't generate heat and you won't be able to use Barrel Stablizer, either, which means you'll be stuck without heated shots and overheat for a long while unless you're somehow able to get onto something's enmity list (This usually only occurs as a result of other players dying from my experience).

NIN also effectively has access to ninjutsu twice as often as a result of this, as they're able to use Hide in combat to refresh their ninjutsu cooldowns and also to avoid having to use Suiton for Trick Attack.

I think that the method that's used to determine whether or not a player is in combat needs to be changed from being on an enemy's enmity list to something else to prevent these things from happening.