on the EPP 5.X it is required to assign a error page to the url as shown below.

http://localhost:8080/portal/private/available-portal/not-available-page

the key issue is when a page is accessed which is actually not present under the available-portal for example http://localhost:8080/portal/private/classic/somepage.html (somepage.html actually doesn't exists) the portal doesn't show 404 rather it returns to the home page http://localhost:8080/portal/private/classic.

Since on 5.1.1 this was not possible, I tried on EPP 5.2
I tried configuring the error page according to https://community.jboss.org/wiki/CustomErrorPages but I was only successful the /portal level
but not in /portal/classic.

I tried modifying controller.xml <route path="/error.html"> to <route path="/classic/error.html"> but this did not help too.
I guess above mentioned approach only works when there is 404 response but in case of /classic/somepage.html 404 is not received.

I tried analysing  http://anonsvn.jboss.org/repos/gatein/epp/portal/tags/EPP_5_2_0_GA/component/web/controller/src/main/java/org/exoplatform/web/WebAppController.java but no avail.