myfaces core12 < v1.2.4 has a bug. a workaround to use extval with these versions is available at http://code.google.com/p/os890/source/browse/trunk/java/web/jsf/extval/myfaces-core12-workaround/CrossValidationMyFacesFixPhaseListener.java
information for users of myfaces-core 1.2.4+ don't need this workaround for these releases.