code: function() {
          var failedChanges = _.first(this._opSetQueue);
          this._opSetQueue = _.rest(this._opSetQueue);
          var nextChanges = _.first(this._opSetQueue);
          AV._objectEach(failedChanges, function(op, key) {
            var op1 = failedChanges[key];
            var op2 = nextChanges[key];
            if (op1 && op2) {
              nextChanges[key] = op2._mergeWithPrevious(op1);
            } else if (op1) {
              nextChanges[key] = op1;
            }
          });
          this._saving = this._saving - 1;
        }
docs: Called when a save fails because of an error. Any changes that were part
  of the save need to be merged with changes made after the save. This
  might throw an exception is you do conflicting operations. For example,
  if you do:
  object.set("foo", "bar");
  object.set("invalid field name", "baz");
  object.save();
  object.increment("foo");
  then this will throw when the save fails and the client tries to merge
  "bar" with the +1.
  @private