This test failed both times:

rx.internal.operators.OperatorMergeDelayErrorTest > testErrorInParentObservableDelayed FAILED
    org.mockito.exceptions.verification.TooLittleActualInvocations: 
    stringObserver.onNext("hello");
    Wanted 2 times:
    -> at rx.internal.operators.OperatorMergeDelayErrorTest.testErrorInParentObservableDelayed(OperatorMergeDelayErrorTest.java:516)
    But was 1 time:
    -> at rx.observers.TestObserver.onNext(TestObserver.java:78)
        at rx.internal.operators.OperatorMergeDelayErrorTest.testErrorInParentObservableDelayed(OperatorMergeDelayErrorTest.java:516)

No idea why or if it's related, but I'd like to understand before merging this.

I don't think MergeDelayError uses ReplaySubject.

I don't fully understand the OperatorMerge, but it contains a lot of mutable variables and synchronized blocks and I'm not sure but I found some anomalies in:

https://github.com/ReactiveX/RxJava/blob/1.x/src/main/java/rx/internal/operators/OperatorMerge.java#L505

It synchronizes on the MergeProducer's this but reads the MergeSubscriber's wip field which generally is accessed through the MergeSubscriber's this. Same seems to be true for the scalarValueQueue.

I've tested things locally and it's working for me.

I opened an issue to track the issue you brought up regarding merge: #1885