Add the lastErrorObject to the .save callback. Now the callbacks are as following.

db.users.insert(document, callback);    // => callback(err, [document])
db.users.save(document, callback);  // => callback(err, document, lastErrorObject)
db.users.update({ ... }, { ... }, callback); // => callback(err, lastErrorObject)
db.users.remove({ ... }, callback); // => callback(err, {n: amountRemoved})
db.users.findAndModify({ query: { ... }, update: { ... }}, callback); // => callback(err, document, lastErrorObject)