repro script / jstest that runs two threads that update the same document: load( 'jstests/libs/parallelTester.js' ); db.foo.drop(); db.foo.insert({}); assert .eq(1, db.foo.count()); var t0 = new ScopedThread(function() { for ( var i=0; i<100; ++i) { var res = db.foo.update({}, { $inc: { x: 1 } }); assert .eq(1, res.nModified, res); } return 'ok' ; }); t0.start(); for ( var i=0; i<100; ++i) { var res = db.foo.update({}, { $inc: { y: 1 } }); assert .eq(1, res.nModified, res); } t0.join();