AQ20 missing loot:

The following three items drop during the General Rajaxx encounter, but not from the actual boss itself:
Gavel of Qiraji Authority
Fury of the Forgotten Swarm
Treads of the Wandering Nomad

This is a missing random trash drop:
Coif of Elemental Fury

ZG incorrect loot:
Warblade of the Hakkari (Off hand) can drop from both Bloodlord Mandokir (9%ish) and Hakkar (4%ish), currently it is only listed as dropping from Bloodlord Mandokir (3,8%) as of 1.5.4.

Also see this post by Nihlo for some additional information on some of this: a bunch of little things