Unable to Access Object

I have client on SharePoint (MOSS) 2007 who reported that there was a particular excel file in one of their document libraries that would never show up in Search results, no matter what keywords or other criteria were used in their search request, including searching by its title or other obvious metadata.
Looking in the Search Crawl Log (Shared Services Administration > Search Settings > Content Sources > Crawl Log), there was an error encountered attempting to index said file: "Unable to Access Object". Unfortunately, a Google search for SharePoint "Unable to Access Object" returned only 3 results, and those were unrelated to Search Indexing (Hopefully Google will pick up this post for anyone else that ever experiences this issue).
So it seemed that the Search Crawl could not Access the file for some reason. I assumed this was somehow permissions related, and spent some time spinning my wheels attempting find the source of the problem, including logging in interactively as the Crawl Service Account. But I seemed to be able to access and open the file without problems with just about all users.
I copied the file to other SharePoint farms, and the same problem occurred: The "Unable to Access Object" error occurred in those environments too, and the document would not appear in search results.
So, now that it was obvious that the problem was following the file, it was clearly not environmental. So I opened up the file and deleted all the content out of it (it had about a dozen worksheets) and saved it back to the library. Voila, the problem disappeared -- so the source of the problem was in the file's contents somewhere. So then I restored the original version of the file (and the problem returned), and began a process of elimination to figure out which piece of content was causing the problem. Finally determining that one of the worksheets had an embedded excel workbook object that, while appeared fine when viewed, would cause excel to crash when clicked on. Deleting this one object allowed the Search Crawl to correctly index the file.
So, apparently, the logged error "Unable to Access Object" does not mean the entire file could not be accessed, it means that there is some embedded object within the file that could not be accessed (though that doesn't explain why SharePoint wouldn't choose to index the content that it could access, or at least its metadata). So, if you ever end up with a file getting this message in your search crawl, check inside the file, there is probably a bad object inside.