Called next when there is no top

This happens very rarely but we have seen the following exception (pulled from a server running 1.6.4). Looking at the code I believe this condition can still happen in 1.8.0:

java.util.concurrent.ExecutionException: java.lang.IllegalStateException: Called next() when there is no top
...
Caused by: java.lang.IllegalStateException: Called next() when there is no top
HeapIterator.next(HeapIterator.java: 77)
WrappingIterator.next(WrappingIterator.java: 96)
MemKeyConversionIterator.next(InMemoryMap.java:162)
SourceSwitchingIterator.readNext(SourceSwitchingIterator.java: 139)
SourceSwitchingIterator.next(SourceSwitchingIterator.java: 123)
PartialMutationSkippingIterator.consume(InMemoryMap.java:108)
SkippingIterator.seek(SkippingIterator.java:43)