I think ive narrowed it down (QTM you owe me 30+ deaths worth of exp :-p)

So far each failed fd is accompanied by a late hit that then causes the mob to "see through fd" and regain agro, which then reagros all the local mobs.

Example:
You cast feign death 3.
1-2 seconds later-> Ancient mummy's /slow cast ability hits you
All mobs in vacinity to you/mob reagro ill post a log or two in a sec


[21:38:22] [21:38] Vivified Trader hits you for <highlight>335</color> damage.
[21:38:32] [21:38] Everyone thinks you're dead.
[21:38:38] [21:38] <RED>You are struck by Shock of Disease for 579 points of damage.
[21:38:39] [21:38] You dodge Crypt Watcher's attack.
[21:38:39] [21:38] You parry Vivified Trader's attack.
[21:38:40] [21:38] Feign Death III has been toggled off.

Thats 6 seconds from feign till the time the spell landed, im not sure of the cast time, but that spell landing is what reagroed me.  Ill try to get a few more for you

heres another
[21:52:04] [21:52] Vivified Trader begins to cast a spell.
[21:52:04] [21:52] Slave of Nusibe begins to cast a spell.
[21:52:04] [21:52] Vivified Trader begins to cast a spell.
[21:52:04] [21:52] Everyone thinks you're dead.
[21:52:08] [21:52] Slave of Nusibe's <highlight>Disease of the Ancients</color> hits you for <highlight>412</color> damage.
[21:52:08] [21:52] Slave of Nusibe's <highlight>Disease of the Ancients</color> hits you for <highlight>412</color> damage.
[21:52:08] [21:52] Slave of Nusibe begins to cast a spell.
[21:52:08] [21:52] Vivified Trader's <highlight>Disease of the Ancients</color> hits you for <highlight>413</color> damage.
[21:52:08] [21:52] Vivified Trader's <highlight>Disease of the Ancients</color> hits you for <highlight>413</color> damage.
[21:52:08] [21:52] Vivified Trader begins to cast a spell.
[21:52:08] [21:52] You parry Crypt Watcher's attack.
[21:52:09] [21:52] <BLUE>Your damage shield hits Crypt Watcher for 40 damage.
[21:52:09] [21:52] Feign Death III has been toggled off.



and a third (right after the previous, seperated log so you can clearly see that casting is breaking the feign
[21:52:08] [21:52] Slave of Nusibe begins to cast a spell.
[21:52:08] [21:52] Vivified Trader begins to cast a spell.
[21:52:10] [21:52] Crypt Watcher hits you for <highlight>400</color> damage.
[21:52:10] [21:52] You parry Awakened Guard's attack.
[21:52:10] [21:52] <RED>You are struck by Shock of Disease for 585 points of damage.
[21:52:10] [21:52] Vivified Trader begins to cast a spell.
[21:52:11] [21:52] You parry Crypt Watcher's attack.
[21:52:11] [21:52] Everyone thinks you're dead.
[21:52:12] [21:52] Slave of Nusibe's <highlight>Disease of the Tomb</color> hits you for <highlight>391</color> damage.
[21:52:12] [21:52] Slave of Nusibe's <highlight>Disease of the Tomb</color> hits you for <highlight>391</color> damage.
[21:52:12] [21:52] Vivified Trader's <highlight>Plague of the Tomb</color> hits you for <highlight>390</color> damage.
[21:52:12] [21:52] Vivified Trader's <highlight>Plague of the Tomb</color> hits you for <highlight>390</color> damage.


Conclusion If a mob starts casting a slow cast ability before you feign, theres a chance they will continue to cast after you feign, when the cast lands, feign is broken and mobs around you and around the casting mob re agro.  If the spell lands 1-2 seconds after FD, then it appears fd failed, eventhough it worked.

Cool beans, any word on if its gonna be fixed or if this is the new intended?  Being as it was like this in eq1 for 4+ years and only once they added stonewall did it cancle (stone wall made it so u had a % chance to remain fd even after being cast on)

Mad props, Fujitsu ! Glad you figured this one out, cause it's annoying as Foxtrot.