Example scenario:

web.xml contains the following block:

...
<error-page>
  <error-code>java.lang.Throwable</error-code>
  <location>/unexpected.jsp</location>
</error-page>
...

This block wrongly contains "error-type" instead of "error-code".
When this application is deployed the error-page is registered as the default error page because org.apache.tomcat.util.descriptor.web.ErrorPage#setErrorCode catches NumberFormatException and assumes errorCode 0.

Then later when a request to a servlet is made which uses the sendError (e.g. with 404) method the page "unexpected.jsp" is displayed instead of the default 404 page.

It's also related to org.apache.catalina.core.StandardHostValve#status(Request request, Response response) where it says:
...
 ErrorPage errorPage = context.findErrorPage(statusCode);
        if (errorPage == null) {
            // Look for a default error page
            errorPage = context.findErrorPage(0);
        }
...

"context.findErrorPage(0)" returns "/unexpected.jsp" although it should return null.