M-x customize strangeness with "Hide" button

[Eli Zaretskii]

I see this in all versions of Emacs 21, including 21.1, the current CVS, 
and the pretest of 21.2, both in the tty version and on X:

  emacs -q --no-site-file
  M-x customize-variable RET truncate-lines RET

(actually, any variable will do).  Now watch closely: there's an extra 
"[Hide]" button to the right of the first line of the doc string.

Bug or feature?  (It looks like a bug to me, but what do I know?)