Run this test in the shell and the final "assert.commandWorked" will fail: (function() { "use strict" ; const st = new ShardingTest({shards: 2, config: 1}); const dbName = "testDb" ; const collName = "testColl" ; const ns = dbName + "." + collName; const db = st.getDB(dbName); // Set up a sharded collection with two shards and a chunk on each. assert .commandWorked(st.s.adminCommand({enableSharding: dbName})); assert .commandWorked(st.s.adminCommand({shardCollection: ns, key: {skey: 1}})); assert .commandWorked(st.s.adminCommand({split: ns, middle: {skey: 0}})); assert .commandWorked(     st.s.adminCommand({moveChunk: ns, find: {skey: MinKey}, to: st.shard0.shardName})); assert .commandWorked(st.s.adminCommand({moveChunk: ns, find: {skey: 1}, to: st.shard1.shardName})); // Inserting with a dollar prefixed field works. assert .commandWorked(db.runCommand({     insert: collName,     documents: [{skey: -1, dollarPrefixedField: { "$date" : "2012-02-29T00:00:00Z" }}] })); // But an update that uses a transaction to move the document across shards fails with: "unknown // operator : $date" assert .commandWorked(db.runCommand({     update: collName,     updates: [{q: {skey: -1}, u: { "$set" : {skey: 1}}, upsert: false }],     txnNumber: NumberLong(1),     lsid: {id: UUID()}, })); st.stop(); })();