PDF created with ID CS5.5 won't open in Acrobat 9

Question

I have a PDF that was created in InDesign CS 5.5 — using the same standard export profiles we use for hundreds of other files — that appears to have no issues opening in Acrobat X Pro, but which will not open correctly in Acrobat 9 Pro. (Mac & Windows versions)
File created on a Mac with CS 5.5, exported with a standard export preset conforming to the PDF/X-4 standard (compatible with Acrobat 7).
PDF opens and displays as it should in Acrobat X. PDF sent to a color proofer running GMG ColorProof 5 RIP also processes normally.
But when trying to open the same PDF file in Acrobat 9 — either Windows or Macintosh — we get the error:
"An error exists on this page. Acrobat may not display the page correctly. Please contact the person who created the PDF document to correct the problem."
Half the type and much of the art (simple bitmap polygons) does not display, as if it were missing. (The file does open, it just opens incorrectly.)
I believe it's unrelated, but maybe worth mentioning that this PDF reported a "Character references .notdef glyph" error, which we occasionally get on some files. This does not create issues on press (it hasn't yet), but it technically outside the PDF/X-4 spec, so it gets flagged. Deleting the type referenced by the error removes the warning, but does not correct this issue, where the page doesn't display.
I've looked through the document properties and run several preflight checks to look for aberrations in the file, anything that's out of spec, and I can't find anything.
Again, there's nothing in this file that isn't in literally hundreds of other files, so I'm at a loss as to what's creating this issue.

Answer 1

an update
after going through the layout in question piece by piece, removing then adding back in each individual item one at a time until the error occurred, I discovered two artifacts in the layout that were the cause. Don't ask me what they were, but removing them solved the issue.
They weren't visible, as they weren't even points, really — as if they were two points of an incompletely deleted object.
My first guess was a corrupted frame, as I've seen that happen a number of times, even going back to Quark 3, where an individual object, or the art placed in it, munges the postscript somehow and then the resulting document. That's why I usually start removing items on the page until the problem goes away, to find the culprit. In this case, they were hard to find as they were single points behind other objects (and were getting selected inadvertently with them).
Anyway, not sure what that was, but it solved my issue.