Took 2 months to track this down, in my spare time. 

Test Environment:
Weblogic 5.1 servlet, Using LoadRunner running 10 concurrent users generating 
the same 17 page pdf over and over with no think time. Pdf has a common small 
gif logo on each page, 5 pages include a common complex chart.

Problem:
An occasional NullPointerException was being thrown from the PDFXObject.output
() method.

Root Cause:
The design of the system did not consider concurrent PDFDocuments accessing the 
same FopImage. The FopImage member of the PDFXObject opens and closes an input 
stream from the image url, as well as nulling out other elements. Access to the 
FopImage member (a GifImage) was unsynchronized.
The line that always threw the execption was
int bitMapsSize = fopimage.getBitmapsSize();

Solution:
In the PDFXObject.output() method, added the following line at the beginning of 
the method body.

synchronized(fopimage) {

The NullPointerExceptions disappeared! 

Please no applause, just send money or thanks to mb32@ntrs.com