If you accidentally delete an appointment book entry on your HP 100/200LX, you can restore it by immediately pressing (CTRL)-(Z) (or (MENU) Edit Restore). If you press (Fn)-(.) to Cut an appointment, you can press (Fn)-(+) to Paste it back.
On the HP 95LX, if you press (F3) (Cut) to delete an appointment, you can press (F4) (Paste) to restore it.