if you change this test here https://github.com/mongodb/mongo/blob/70b842700570157b1b0cb49fb503608c4cf2ec6c/jstests/noPassthrough/out_drop_temp_collection.js#L48 to drop the source collection the test fails because the aggregation succeeds. even if you change the wait point to wait until after the source collection has been accessed. This could lead to the collection being recreated or partial results in the output collection.