The problem occurs when an exception (eg: NullPointerException) is thrown during the rendering of a page with jsp template: if a custom error page (also with jsp template) has been configured in web.xml, it does not display (blank page instead).

Example of web.xml configuration:

<error-page>
  <exception-type>java.lang.Throwable</exception-type>
  <location>/myapp/errors/error.html</location>
</error-page>

The MgnlMainFilter receives the request for the error page, but immediatly bypasses it, due to voter "dontDispatchOnForwardAttribute":

2010-03-04 10:57:32,437 DEBUG info.magnolia.cms.filters.MgnlMainFilter MgnlMainFilter.java(doFilter:93) Handling URI: /myapp/errors/error.html - Path info: null
2010-03-04 10:57:32,437 DEBUG info.magnolia.voting.DefaultVoting DefaultVoting.java(vote:58) voter [dontDispatchOnForwardAttribute: ] fired 1
2010-03-04 10:57:32,437 DEBUG info.magnolia.voting.DefaultVoting DefaultVoting.java(vote:62) highest vote is now 1

I found that the dontDispatchOnForwardAttribute voter prevents the rendering of the error page because just before starting the rendering of the original page (the one that throws the Exception) an attribute DONT_DISPATCH_ON_FORWARD_ATTRIBUTE was put in request by the JspTemplateRenderer:

// set this attribute to avoid a second dispatching of the filters
request.setAttribute(DontDispatchOnForwardAttributeVoter.DONT_DISPATCH_ON_FORWARD_ATTRIBUTE, Boolean.TRUE);

JspTemplateRenderer should remove that attribute when catching the exception: this would fix the problem.
See attached patch: JspTemplateRenderer.patch