Random "IllegalStateException: TimestampCombiner moved element from X to Y" seen in running topology

We have a streaming job that does some aggregation, it looks something like:
```

- Read from Pub/sub (use timestamps from pubsub)
- Window into 4 hour fixed windows, no allowed lateness
-
CombineByKey with some aggregations
- Write out
```

Today, the pipeline stalled, continuously throwing an exception I had never seen before (it had been running fine for ~10 days previously)
```

java.lang.IllegalStateException: TimestampCombiner moved element from 2019-02-28T08:00:05.000Z to earlier
time 2019-02-28T07:59:59.999Z for window [2019-02-28T04:00:00.000Z..2019-02-28T08:00:00.000Z)
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.WatermarkHold.shift(WatermarkHold.java:117)
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.WatermarkHold.addElementHold(WatermarkHold.java:154)
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.WatermarkHold.addHolds(WatermarkHold.java:98)
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.ReduceFnRunner.processElement(ReduceFnRunner.java:605)
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.ReduceFnRunner.processElements(ReduceFnRunner.java:349)
org.apache.beam.runners.dataflow.worker.StreamingGroupAlsoByWindowViaWindowSetFn.processElement(StreamingGroupAlsoByWindowViaWindowSetFn.java:94)
org.apache.beam.runners.dataflow.worker.StreamingGroupAlsoByWindowViaWindowSetFn.processElement(StreamingGroupAlsoByWindowViaWindowSetFn.java:42)
org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowFnRunner.invokeProcessElement(GroupAlsoByWindowFnRunner.java:115)
org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowFnRunner.processElement(GroupAlsoByWindowFnRunner.java:73)
org.apache.beam.runners.dataflow.worker.repackaged.org.apache.beam.runners.core.LateDataDroppingDoFnRunner.processElement(LateDataDroppingDoFnRunner.java:80)
org.apache.beam.runners.dataflow.worker.GroupAlsoByWindowsParDoFn.processElement(GroupAlsoByWindowsParDoFn.java:134)
org.apache.beam.runners.dataflow.worker.util.common.worker.ParDoOperation.process(ParDoOperation.java:44)
org.apache.beam.runners.dataflow.worker.util.common.worker.OutputReceiver.process(OutputReceiver.java:49)
org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.runReadLoop(ReadOperation.java:201)
org.apache.beam.runners.dataflow.worker.util.common.worker.ReadOperation.start(ReadOperation.java:159)
org.apache.beam.runners.dataflow.worker.util.common.worker.MapTaskExecutor.execute(MapTaskExecutor.java:76)
org.apache.beam.runners.dataflow.worker.StreamingDataflowWorker.process(StreamingDataflowWorker.java:1233)
org.apache.beam.runners.dataflow.worker.StreamingDataflowWorker.access$1000(StreamingDataflowWorker.java:144)
org.apache.beam.runners.dataflow.worker.StreamingDataflowWorker$6.run(StreamingDataflowWorker.java:972)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
java.lang.Thread.run(Thread.java:745)
```

I'm not really sure what could have happened here, it looks like the element is trying to be put into the wrong window?  This is running on dataflow with "enable_streaming_engine" on.

Imported from Jira [BEAM-6757](https://issues.apache.org/jira/browse/BEAM-6757). Original Jira may contain additional context.
Reported by: SteveNiemitz.