I've only had success in reproducing this issue with the MMAPv1 storage engine, and not with the WiredTiger or EphemeralForTest storage engines; however, it isn't clear to me why this issue would be storage engine-specific though. python buildscripts/resmoke.py --suites=no_server repro_server31019.js --storageEngine=mmapv1 --repeat=5 repro_server31019.js ( function () { "use strict" ; const verbositySettings = tojson({ verbosity: 1, replication: 2, storage: 2, }); const rst = new ReplSetTest({ nodes: 1, nodeOptions: { setParameter: {logComponentVerbosity: verbositySettings}, } }); rst.startSet(); rst.initiate(); const primaryDB = rst.getPrimary().getDB( "test" ); rst.add({ setParameter: { "failpoint.initialSyncHangBeforeCopyingDatabases" : tojson({mode: "alwaysOn" }), logComponentVerbosity: verbositySettings } }); // We disallow the secondary node from voting so that the primary's featureCompatibilityVersion // can be modified while the secondary node is still waiting to complete its initial sync. { const replSetConfig = rst.getReplSetConfigFromNode(0); replSetConfig.members = rst.getReplSetConfig().members; replSetConfig.members[1].priority = 0; replSetConfig.members[1].votes = 0; ++replSetConfig.version; assert.commandWorked(primaryDB.adminCommand({replSetReconfig: replSetConfig})); } // We set the primary's featureCompatibilityVersion to "3.4" and implicitly create a collection // without a UUID via an insert operation. { assert.commandWorked(primaryDB.adminCommand({setFeatureCompatibilityVersion: "3.4" })); primaryDB.mycoll.drop(); assert.writeOK(primaryDB.mycoll.insert({_id: " while in fCV=3.4" })); } // Next, we set the primary's featureCompatibilityVersion to "3.6" and drop the collection that // was previously created. We then implicitly create another collection of the same name (but // with a UUID this time) via an insert operation. { assert.commandWorked(primaryDB.adminCommand({setFeatureCompatibilityVersion: "3.6" })); primaryDB.mycoll.drop(); assert.writeOK(primaryDB.mycoll.insert({_id: " while in fCV=3.6" })); } // Finally, we allow the secondary node to proceed with its initial sync. It should end up with // only the document that was inserted into the collection when the primary's // featureCompatibilityVersion was "3.6" . const secondaryDB = rst.getSecondary().getDB( "test" ); assert.commandWorked(secondaryDB.adminCommand({ configureFailPoint: "initialSyncHangBeforeCopyingDatabases" , mode: "off" , })); rst.checkReplicatedDataHashes(); rst.stopSet(); })();