Some flavours of subscribe seem to ignore this guideline in the case that the given final Object onNext is null, for example: (Observable.java:366)

public Subscription subscribe(final Object onNext, final Object onError) {
    // lookup and memoize onNext
    if (onNext == null) {
        throw new RuntimeException("onNext must be implemented");
    }
    final FuncN onNextFunction = Functions.from(onNext);

This behaviour could be deliberately chosen, but then I wonder why other flavours of subscribe (those where an Action1<T> is passed) do neatly follow this guideline and only throw when onNext is actually being called: (Observable.java:428)

        public void onNext(T args) {
            if (onNext == null) {
                throw new RuntimeException("onNext must be implemented");
            }
            onNext.call(args);
        }

Moreover, I would actually expect both cases to call onError instead of throwing an Exception, though this could perhaps be defended, and for sure you would want to fix issue #198 first. It seems strange to me however that there is this difference in behaviour between passing an Object and passing an Action1 for onNext, or am I missing something?