The default error pages in tomcat7 displays information about the hosting system. And before version 7.0.29 there was not much one could do about it (unless using tomcat 5 or 6).
The way of hiding such information was to use custom error-handlers defined in context or webapp. In tomcat7 it was not possible (beacuse of confusion regarding the servlet 3.0 spec), but it was reported as a bug https://issues.apache.org/bugzilla/show_bug.cgi?id=52135 and then fixed for version 7.0.29.
In my mind this is a security feature so why isn't the version on Ubuntu 12.04 patched with this fix?