If the "update" option is set on the JAR task,  the target jar will be updated 
even if an update is not required (ie the src files passed to the jar task are 
at the same level as the files in the target jar).  

I have done an initial investigation of this problem below are my findings:

-When the update option is set the Zip task renames the zipFile to a tmp file 
before the call to isUpToDate(...) is made.  The isUpToDate method is passed 
the original zipFile name.  The original file no longer exists because the file 
was renamed.  Therefore when Jar.isUpToDate trys to find the ZipFile it no 
longer exists and the following exception is thrown: 
java.util.zip.ZipExceptionThe system cannot find the file specified.  The 
exception flags the Jar task to update the jar.

-This bug can be fixed by moving the isUpToDate call above the zipFile rename 
in the ZIP task.