The test may fail in sharded passthrough mode in some cases. For instance, in a test that verifies 'whenMatched: fail, whenNotMatched: insert' mode we expect the test to return a DuplicateKey error, but also insert some documents in the target collection. The test was written with the assumption that documents in the source collection will be always processed in the same order as they were inserted into the collection. In a sharded cluster this assumption is wrong.