protected O handleFilterException(I inMesg, ZuulFilter<I, O> filter, Throwable ex) {
        inMesg.getContext().setError(ex);
        if (filter.filterType() == FilterType.ENDPOINT) {
            inMesg.getContext().setShouldSendErrorResponse(true);
        }
        recordFilterError(inMesg, filter, ex);
        return filter.getDefaultOutput(inMesg);
    }
This is typically set by a filter when wanting to reject a request and also reduce load on the server by
 not processing anymore filterChain
zuul-core/src/main/java/com/netflix/zuul/netty/filter/BaseZuulFilterRunner.java