one already can see the shifting of page items on all right side pages with CS5, CS5.5 and CS6.
With InDesign CS4 one can see a shift also on left side pages.
The script you linked to was written and tested for CS2. That's quite a while.
Maybe something has changed between CS2 and CS4. Maybe not.
After a bit of testing with several documents and several versions of inDesign I found, that:
1. The amount for shift of the page items is not a random thing.
2. The shift depends on the values for the inner and outer margins of the right pages (or with CS4 also on the left pages).
I could make out two rules:
A. If the inner and the outer margins are the same no shift will occur.
B. If the margin values are different some shift will happen.
How big is the amount of shifting?
It's margin left minus margin right on a right side page.
CS4 only as far as I can see: It's margin right minus margin left on a left side page.
And an extra observation I did:
It will not occur on the first page of a document, if that page is right from the spine.
CS4 only as far as I can see: It will not occur on the second page, if it is left from the spine.
Note: I did not test with documents with more than one page right or left from the spine.
So with this all in mind we could rewrite Harb's script snippet to counter that phenenomen.
The source code of the snippet is open. Maybe someone already did a rewrite…
All my testing was with German OSX 10.6.8 and 10.7.5.
I did not yet test with InDesign CS3, CC, CC-2014 and CC-2015.
Also did the same tests with CC v9.3.0, CC 2014.2 and CC 2015.4.
Same problem like CS5, CS5.5 and CS6.
Finally I tested with CS3 and I could not observe any shifting pageItems.
So, the bug, let's call it a bug, sneaked already in with CS4. And was only fixed half (left hand sided pages) with CS5.
So one could work around the problem by changing the inner and outer margins to one single value before running the script.
Thanks!! I'll probably just change the margin values for now. I'll need to learn a lot more about coding before I can actually fix the problem