If the resource class hierarchy looks like the one described below, then a GET with no Accept header does not attract the expected response with Content-Type determined by the @Produces annotation.

AnnotationFreeSubResource -implements> SubInterface --extends-> RootInterface
\
+-extends> AbstractAnnotationFreeResouce --implements-> RootInterface

where the following annotations are present:

RootInterface - @Produces on interface, @GET and @Path on separate methods
SubInterface - @Produces on interface, @POST on a single method
AbstractAnnotationFreeResouce - no direct annotations, implements @GET method inherited from RootInterface
AnnotationFreeResouce - no direct annotations, implements @Path method from RootInterface and @POST method from SubInterface

The underlying problem is the same as for RESTEASY-426.