Guild Wars 2

Just wondering what’s up with this. Occasionally / quite often, random creatures like wargs and harpies suddenly become immune to damage (but not Determined or anything of the sort), and if you try to hit ‘em it’ll say ‘Invulnerable’. They stop fighting at this point too. Stays like that for about 10 secs after which they’re no longer invincible and start fighting back too. It’s kind of like the hp regen npc’s receive when they stray too far from their spawn point and retreat, except there’s no straying in this case at all.
So what’s this then?

The server is losing track of or misplacing you, so the mobs can no longer reach you and become invulnerable to prevent exploitation. It happens more often in the water, and is very annoying.

Ye, Like excretus is saying. What I’ve also noticed is that hightdifference can cause this (again specially in the water). I suspect that the map is ’ layered’ And that if your at the bottom of one layer and the enemy’s is at the top of a lower layer, the damage won’t get there untill you or the mob moves a bit