From time to time we have users report problems when dealing with Oracle databases either with using the DB Explorer, reverse engineering or some other function. The issue almost always stems from the fact that they are using the old classes12.zip driver and not the newer ojdbc14.jar driver (JDBC for java 1.4/5.0). The newer driver is much more robust and will work with Oracle DB versioned 8.1.7 or greater and can be downloaded from here.
Additionally there is a known problem when using the Oracle 9 or 10 JDBC driver in an international environment that will result in a stack trace like:
java.sql.SQLException: ORA-00604: error occurred at recursive SQL level 1
ORA-12705: Cannot access NLS data files or invalid environment specified
The fix is to upgrade your JDBC driver to the latest version, available here.