I have been using Fop-0.18.1-DEV for a while now and have encountered some 
problems with the PDF generated.   The most concrete of these is that Adobe 
Acrobat converter for the Palm refuses to convert seemingly "any" complex Fop-
generated file, claiming that it has a bad page contents.   (There doesn't seem 
to be a version number for this converter, but it's the "Public Beta" for 
Acrobat Reader 5.0, available from 
http://www.adobe.com/products/acrobat/readerforpalm.html)
The full error message is:
"Error converting to tagged PDF: Could not read page structure (bad page 
contents) [3]."   (where the 3 is the page on which it encountered the problem).

The FO for the simplest problem example I can generate problems with is 
available as http://users.breathemail.net/njr/fop/r.fo, and the PDF generated 
by fop is there also as http://users.breathemail.net/njr/fop/r.pdf.   This FO 
was generated using XT, but I don't imagine that is very relevant.

Obviously this could be a problem with the Acrobat converter, but I am assuming 
that Adobe effectively acts as the "gold standard" for PDF, and it is 
explicitly claiming that there is an error in the PDF.   I will, however, 
submit the bug to Adobe too and reference this bug report.