Scheduling should serialize notifications.

[Dorus]

Currently, onError cuts ahead of onNext calls when you schedule work, but does not (cannot) when you use a single thread.

Observable.create(o -> {
    o.onNext(1);
    o.onError(new NotImplementedException());
})
.observeOn(Schedulers.newThread())
.subscribe(i -> System.out.println(i), 
        e -> System.out.println("error:" + e));

This yields error instead of 1, error as expected. Other implementations of Rx like Rx.Net do serialize notifications. This 'cut ahead' behavior is actually described as a common pitfall in RxJava by some.

[akarnokd]

RxJava 2.0 will allow you to customize this behavior in many operators, including observeOn, delay, combineLatest, zip, groupBy, onBackpressureBuffer, skipLast and takeLast.

I was planning to post a PR targeting observeOn to remove some unnecessary code (#3002) and may well do this as well.

[Dorus]

This is really scary nondeterministic behavior, and against Rx contract. I hope the (future) default will be to not lose data. If anything it might be an appropriate setting on the subscriber.

[akarnokd]

The observeOn operator received an overload that takes a delayError parameter you can set.

Closing via #3682.