I also noticed, if you wanna save your data using the model (by code).
the new items are not saved correctly, due to the "user state", what "remember" the last item manipulated by the user.
So the new item data you would save will be "mixed" with the LAST item used by the user.
so, here my workaround:
add this function to the the following file:
ROOT_YOUR_SITE\administrator\components\com_your_component\classes\jmodel.item.php
public function saveNew($data)
{
$this->setState($this->getName() . '.id', 0);
parent::save($data);
return true;
}
when you wanna save a new item, using the MODEL, just call this new function.