I report two problems, but they really are the same bug.

1st problem:
From emacs -Q :
select e.g. the first line of the scratch buffer
hit C-x n n
(that is a disabled command, so you get a prompt)
answer 'n' to the prompt
the buffer is narrowed anyway.

I expected nothing to happen wrt narrowing.

2nd problem:
From emacs -Q, eval :

(defun my-command ()
  (interactive)
  (message "Got called !"))
(put 'my-command 'disabled t)

then:
M-x my-command RET
(you get a prompt)
hit SPC to execute just once and leave disabled
hit C-h e to show the *Messages* buffer
You can see:
Got called ! [2 times]

I expected to see "Got called !" (only once).

Both of these seem to be fixed with the attached patch.

However, with this patch, when `disabled-command-function' decides to
run the command, it does so by invoking call-interactively, thereby not
executing what command-execute would have done if the command had not
been disabled. It doesn't seem right but I'm not sure (and have no idea
what to change anyway).

Here's the patch :