Starting with the pre-prequels (because there are only four references), we have:
From The Butlerian Jihad, two in text uses, one glossary ref:
"I am intrigued by questions. Why do wealthy humans eat with such ceremony? Why do they consider certain foods and beverages superior to others, when the nutritional value is the same?" The robot's voice became more erudite. "The answer, Omnius, has to do with their brutally short lives. They compensate with efficient sensory mechanisms capable of imparting intense feelings. Humans have five basic senses, with countless gradations. The taste of Yondair beer versus Ularda wine, for example. Or the feel of Ecaz burlap compared with parasilk, or the music of Brahms versus—"
On the opposite shore, at the widest part of the delta, he visited a bustling river market. Rafts and barges had been lashed together for so long that they might well have been part of the landscape. The merchants' quarter was not far from the Starda spaceport, where vendors provided offworld oddities: drugs from Rossak, interesting woods and plants from Ecaz, gems from Hagal, musical instruments from Chusuk.
Ecaz — Unallied Planet
And from The Machine Crusade, one ref:
Iblis remained on Poritrin for a month and arranged a series of private meetings with noblemen, some of whom had journeyed from Ecaz and other League Worlds for the belated festival. Despite the gravity of the machine threat, the patricians were in no mood to discuss serious matters. They wanted to savor their victory for a while, though it was only a small step toward the ultimate goal. Dealing with these fools, Iblis finally reached a peak of frustration, and announced that he would he leaving in order to oversee the important matters of the Jihad.
So far so good, right, no problems? WRONG.
Remember what FH wrote in the "Terminology of the Imperium" in Dune?
ECAZ: fourth planet of Alpha Centauri B; the sculptors' paradise, so called because it is the home of fogwood, the plant growth capable of being shaped in situ solely by the power of human thought.
Hmm...Alpha Centauri B, huh? Let's see, there was something important about Alpha Centauri, but I can't quite seem to remember what it was....Can anyone remember?
OH YEAH! It's right next door to Earth, in galactic terms. Only 4.2 light years away.
Seems kinda odd, doesn't it, that Earth Omnius would have left an unconquered human world so close? The League worlds, not originally conquered by the Titans, were supposed to be on the fringes of the Old Empire. Mighty convenient to have the boundary running so close, huh?
Anyone seen a rationalization for this, or did they not even notice their screw up?
(More to come)