Guild Wars 2

Earth Elementals have the ‘Stability’ boon which states that they are immune to cripple and chill. Yet as an Elementalist I am quite able to cripple and chill them (skill 4 Earth and skill 4 Water).

Their movement speed slows and the cripple/chill debuff icon shows up under their names.

It is because stability does only work against “control effects”…
http://wiki.guildwars2.com/wiki/Control_effect

chill, cripple & immobilize bypasses stability, it doesn’t make you “loose control” in the same sense… don’t know why, been like this since early betas, it’s not just elementals either, it’s everything that has stability.

Ofcourse there are three things that doesn’t really abide to that rule;
the white buff called “Unshakable”, blue “Determined” and that flaming “Righteous Indignation”

So… not a bug, merely a feature.