The error page for a FlexWiki application should have a custom error page defined in the web.config file

 <!--  CUSTOM ERROR MESSAGES
   Set customErrors mode="On" or "RemoteOnly" to enable custom error messages, "Off" to disable. 
      Add <error> tags for each of the errors you want to handle.


      "On" Always display custom (friendly) messages.
      "Off" Always display detailed ASP.NET error information.
      "RemoteOnly" Display custom (friendly) messages only to users not running 
      on the local Web server. This setting is recommended for security purposes, so 
      that you do not display application detail information to remote clients.-->
 <customErrors mode="RemoteOnly" defaultRedirect="default.aspx/<def_namespace>/CustomErrorPage.html" />

Create a CustomErrorPage Topic

The second step is to create a page to display when an error occurs. An example of such a page is at OdsWikiErrorPage This page does not give out a lot of detail, but it does tell the user that the error has been logged and can be investigated if the error is persistent (well, they have to read what is on the page to determine that). Also note the use of the Title property to change the displayed title

The link below will produce an error and display the OdsWiki Error Page:

http://ods.dyndns.org/FlexWiki/blade.aspx?topics=HomePage&property=_Body

The correct link with no error is:

http://ods.dyndns.org/FlexWiki/blade.aspx?topics=HomePage&properties=_Body