Signet of Spite cooldown bug
(Necromancer bug)
To observe that bug you have to do as following: Equip in a utility slot signet of spite. Use it on a mob, then get out of combat. Change the utility skill next to the signet (right or left), and watch the skill you just changed have the same cooldown as your signet of spite. From testing this happens only with signet of spite cooldown.
Happens with any utility skill on any profession, actually. It may just be most reproducible with Signet of Spite in your experience. Frequently noticed with Signet of Undeath for me, but I have seen it on all professions.
What better than getting 180cd on corruption xD