In order to compute parent documents based on a child document, ReverseNestedAggregator does the following:

int parentDoc = parentDocs.advance(childDoc);

But the behavior of advance is undefined when the target is less than or equal to the target document, which can happen if you have 2 matching child documents that have the same parent.

This works fine in most cases when the filter cache is enabled since FixedBitSet is permissive and would go the the next set bit after childDoc. But if the filter cache is disabled then you might run into trouble.