In helping one of my colleagues this week I've come across another common problem that should be audited by everyone running a J2EE app server; JDBC driver version.
1. If you're application has been working and all of the sudden starts to have strange SQL errors it never had before
2. Your application works with intermittent SQL errors (unrelated to SQL statement bugs).
3. See a lot of StaleConnectionExceptions
Check the version of the database server and then the version of the JDBC driver (in WebSphere Application Server the JDBC driver version is printed in SystemOut.log during the app server startup sequence). Typically the DBA just updates the server without telling anyone. This means that a bunch of clients are backlevel on the JDBC driver. I don't know what it is that some database vendors do in their fixpacks but they often seem to break the protocol used by the previous client drivers. So... audit your JDBC drivers periodically and if you can get your DBA to notify you of when updates are going on to the database servers you could even save yourself some grief in production.