Debian sudo components have been built with Ubuntu semantics, i.e. the user's password is required rather than root's as would be expected from Debian & other Unix variants from way-back.

This leads to some odd behaviour.  /usr/bin/su-to-root works OK & is my preferred option on menus & follows Unix standard.  gksudo, which from the name, seems to be a Gnome program and should be part of the basic install follows Ubuntu but gksu seems to follow Unix.  KDE programs with an administrator button need Ubuntu.  It looks as if the problem might be a library used by sudo, gksudo & KDE programs.

On occasion I've seen both passwords refused.  On others giving the wrong password leads to some transient error messages about the mouse not being able to get focus and possibly someone else may be trying to take over the computer.  It's difficult to pin down some of this behaviour as it seems to be influenced by passwords being retained for a session.