Section 3.2 of the JAX-RS 2.1 Spec reads:

A WebApplicationException thrown during construction of field or property values using any of the 5 steps listed above is processed directly as described in Section 3.3.4. Other exceptions thrown during construction of field or property values using any of the 5 steps listed above are treated as client errors: if the field or property is annotated with @MatrixParam, @QueryParam or @PathParam then an implementation MUST generate an instance of NotFoundException (404 status) that wraps the thrown exception and no entity; if the field or property is annotated with @HeaderParam or @CookieParam then an implementation MUST generate an instance of BadRequestException (400 status) that wraps the thrown exception and no entity. Exceptions MUST be processed as described in Section 3.3.4.

https://issues.redhat.com/browse/RESTEASY-1364 addressed an issue about the part that’s after the first sentence. Unfortunately, it broke the first sentence:

A WebApplicationException thrown during construction of field or property values using any of the 5 steps listed above is processed directly as described in Section 3.3.4.