The undo function has changed for R5.
In previous releases of Notes, undo would only remove text entered since the last save command. For example:
2. Typed paragragh 1,
3. Saved the document,
4. Typed paragragh 2 and then chose Edit->Undo, only paragragh 1 would be removed. Paragraph 2 would remain.
2. Type paragragh 1,
3. Save the document,
4. Type paragragh 2 and then chose Edit->Undo, both paragraghs are removed.