load('jstests/libs/parallelTester.js'); // for ScopedThread   var t = db.getSiblingDB("upsert_bug").getCollection("col");   t.drop();   var upsertFn = function() { for (var i = 0; i < 100; ++i) { var c = db.getSiblingDB("upsert_bug").getCollection("col"); var x = c.update({_id:3}, {$set: {a:'123', b: Math.random()}}, true) assert(!x.getWriteError(), tojson(x)); c.remove({"_id":3}); } }   var threads = []; for (var i = 0; i < 2; ++i) { var thread = new ScopedThread(upsertFn, db); threads.push(thread); thread.start(); }   threads.forEach(function(t) { t.join(); }); Result: ./mongo < ~/tmp/upsert_bug.js MongoDB shell version: 3.0.5 connecting to: test true true connecting to: test connecting to: test assert failed : { "nMatched" : 0, "nUpserted" : 0, "nModified" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 duplicate key error collection: upsert_bug.col index: _id_ dup key: { : 3.0 }" } } Error: assert failed : { "nMatched" : 0, "nUpserted" : 0, "nModified" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 duplicate key error collection: upsert_bug.col index: _id_ dup key: { : 3.0 }" } } at Error (<anonymous>) at doassert (src/mongo/shell/assert.js:11:14) at assert (src/mongo/shell/assert.js:20:5) at ____MongoToV8_newFunction_temp (<anonymous>:5:5) 2015-08-18T16:34:39.075-0700 I - js thread raised js exception: Error: assert failed : { "nMatched" : 0, "nUpserted" : 0, "nModified" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 duplicate key error collection: upsert_bug.col index: _id_ dup key: { : 3.0 }" } } at Error (<anonymous>) at doassert (src/mongo/shell/assert.js:11:14) at assert (src/mongo/shell/assert.js:20:5) at ____MongoToV8_newFunction_temp (<anonymous>:5:5) at src/mongo/shell/assert.js:13 bye