If a new item is added you have to fill the fields and you cannot undo the action.

Steps to reproduce the bug:
1) create a node type, call it "type_a"
2) create a second node type, call it "type_b"
3) add a "node reference" field with "autocreate" option. Configure it with the following settings:
- "Content types that can be referenced": "type_a"
- "Number of values": "unlimited"
Now create a new "type_b" node, fill the first reference node field and press "add another item".
The node will not be saved until you will fill the new referenced node.

I can suggest a solutions:
1) if any field of a line is filled, the refernced node shouldn't be created
2) add a "X" button for delete a line... this would be useful to remove a referenced node (new feature)

I'm not sure if it is a nodereference_field or a nodereference_autocreate issue. So I'm double posting this report.

Thanks all :-)