CDI 1.0 section 10.4.2 Declaring an observer method states:
"Interceptors and decorators may not declare observer methods. If an interceptor or decorator has a method with a parameter annotated @Observes, the container automatically detects the problem and treats it as a definition error."

There is no definition error detected for interceptor and decorator with observer method. However such observer methods are noop.

TCK tests for this scenario (CDI 1.1 master branch):
org.jboss.jsr299.tck.tests.interceptors.definition.broken.observer.InterceptorWithObserverMethodTest
org.jboss.jsr299.tck.tests.decorators.definition.broken.observer.DecoratorWithObserverMethodTest