A zero-percent colwidth causes FOP to run forever when attempting to generate PDF. Yes, I agree that 
<colspec colwidth="0*"/> does not make much sense, but it's unfriendly for FOP to never terminate as 
a result of this. (I discovered this problem as a result of debugging output from a table-generator that 
had a bug.)

Sample file to demonstrate the problem:

<!DOCTYPE section PUBLIC
          "-//OASIS//DTD DocBook XML V4.3//EN"
          "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">
<section>

 <title>
  XXX
 </title>

 <informaltable>
   <tgroup cols="2">
     <colspec colwidth="50*"/>
     <colspec colwidth="30*"/>
     <tbody>
       <row>
         <entry>
          XXX
         </entry>
         <entry>
          XXX
         </entry>
       </row>
     </tbody>
   </tgroup>
 </informaltable>

</section>

As shown, the file processes through FOP okay. However, if you change "50*" to "0*" in the first 
<colspec> line, FOP doesn't terminate. (Strictly speaking, I don't know if it runs *forever*, but it 
certainly runs a very long time.)