Steps to reproduce
The ExistingActionRouteConstraint is there to validate that if there is an action in the url then the controller must have an matching action.
However when a partial router has routed to some other object instance then the validation is still performed against the "original" cms content instead of against the partial routed object.
See
http://world.episerver.com/Modules/Forum/Pages/Thread.aspx?id=82127&epslanguage=en
