One of our customers running Tomcat 4.1 with a small web application we sold them (about a dozen servlets and a similar number of simple JSPs, no large memory demands (few KB per session) sometimes gets OutOfMemoryErrors. Lately it's been getting worse, sometimes the problem starts after only a dozen or so requests.
All JSPs had been compiled weeks ago (and the server since restarted) so it's unlikely to be the known Jasper leak.
Does your customer install/reinstall lot applications to tomcat?
One situation which i know is application, which uses hibernate and cglib. Every time you reinstall application to tomcat, it "leaks" little memory, because hibernate/cglib generates new classes runtime, and this "leaks" little memory, because there is no way to remove class instance from JVM.