NotificationLite.ErrorSentinel.toString

It should delegate the toString to the Exception, not reach into the message. Some Exceptions don't have a message so this could print "null" and not show the exception type.
Unsubscribe eagerly and drop queued onNext to emit onError

The queue will be drained to find and emit the onError.
Assert onError cuts ahead of onNext

... and doesn't allow intermittent onNext via a race of draining the queue and emitting (the draining thread must be where the skipping happens, not the producer thread).
Don't allocate an empty ArrayList for each Observable.empty call
Merge pull request #1687 from jbripley/opt-empty

Don't allocate an empty ArrayList for each Observable.empty call
Merge pull request #1705 from edenman/1.x

Issue #1642 Fix null-emitting combineLatest
Slightly cleaner code for onNext from buffer
Merge pull request #1713 from benjchristensen/tweak-pull-1705-fix

Slightly cleaner code for onNext from buffer
Merge pull request #1683 from benjchristensen/observeOn-error

ObserveOn Error Handling
Added a unit test to demonstrate regression in Observable.mergeDelayError
Fix the failing Observable.mergeDelayError synchronous error unit test
Fix compose generics
Fix Rx serialization bug in takeUntil again
Fix the race condition in BufferUntilSubscriber
Use EmptyObserver instead of Subscribers.empty()

onError should be ignored after unsubscribe is called.
Merge pull request #1714 from benjchristensen/merge-20

Merge 0.20.5 into 1.x
Remove Observable.Parallel

see https://github.com/ReactiveX/RxJava/issues/1673
Merge pull request #1716 from benjchristensen/remove-parallel

Remove Observable.Parallel
0.20.5 and 1.0.0-rc.4
Bug in the onBackpressure operators