I just got jumped by 2 boreworms. Fantastic, I needed more masks. I chase off one, it digs away, kill the other. I wait for the first to return and on cue, combat starts, but when I mash my attack button, and find my character swinging at empty air. Still manage to kill it. I right click where it died to no avail. Log out and log back in without closing the game, and find myself looking at two boreworm corpses, working as intended.
No harm no foul here but dying because you were blocked by an invisible foe, or being unable to agro it, would suck. Running Amber 1.8.