Whenever a user enters wrong data on a node creation/edit form (e.g. uploading an image of wrong size, entering too many values in a limited taxonomy field, etc.) the node will not be created but the node creation/edit form will be shown again and the wrong fields will be marked with a red border indicating that the user entered wrong data. However, there is no error message displayed along with it (e.g. "your images was too small", or "you entered too many tags, please enter only one") and the user will not recognize the red border (even if he recognizes it, he would nevertheless ask what went wrong because no further information is provided).