The following steps create a corner panel whose right-click menu can't, as
far as I can tell, be easily accessed.  The steps should be started with
the bottom left corner of the screen free of panels.

These steps apply to the GNOME2 panel, version 2.0.9 (from the Debian
experimental GNOME2 package, 2.0.9-2).

1) New Panel -> Corner Panel (from another panel).
2) Place the new panel in the bottom left with horizontal orientation; size
it to Medium (48 pixels).
3) Right click -> Properties in the new panel.
4) Clear "Arrows on hide buttons" and "Show hide buttons".
5) Right click -> Add to Panel -> GNOME Menu
6) Right click (at far right edge of panel to avoid foot) -> Add to Panel
-> Laucher from menu -> Other -> Address Book
(actually, I think any application launcher works in step 6; I've also
reproduced with Frozen Bubble and XMMS).
(an alternate step 6 is to add an application launcher by right-clicking on
a Foot -> Applications menu item; the outcome is the same).

At this point you have a panel which, as far as I can tell, can't be
configured at all; there is nowhere to right-click to bring up the
Properties dialog.  Right clicking anywhere opens the right-click menu for
one of the two things contained in the panel, rathern than the right-click
menu for the panel itself.

I was able to find an interesting workaround:
1) Add a second app launcher
2) Move the new launcher to the right edge of the panel (middle-drag)
3) Remove the new launcher from the panel

Immediately after you remove the second launcher from the panel, there will
be a narrow strip at the far right edge of the panel which will yield the
panel menu on a right click.

An alternate workaround is to remove everything from the panel, since
panels have a minimum size; however that's kind of a pain if you want to
reconfigure an existing panel without deleting everything in it.

This problem doesn't appear to be reproducible with only one item in the panel.

Even if a narrow clickable strip is restored, it isn't too user-friendly. 
Some other possibilities are:
-- Pass Shift+RightClick, Control+RightClick, or some more esoteric
combination through to the panel even if the click was on an applet.  I
tried a bunch of these combinations when I first ran into the problem.
-- A keyboard-only shortcut to pop up the panel right-click menu for the
panel under the mouse pointer, similar to the current Alt-F1.

There may already be some such shortcut but I couldn't find it anywhere.