On Thursday 10 January 2008, Richard Stallman wrote:
> Can you provide a complete test case, including an Rmail file
> we can operate on?  I don't have a way to generate a message
> to test this on

Sorry, my bad, happens on any mail I try to 
rmail-redecode-body. e.g. "redecoding" iso-8859-1 a mail that was 
iso-8859-1 before rmail internalised it still gives the same 
error.

Attached please find a RMAIL.redecode file with a single iso-8859-1  
message.  Steps to reproduce (still only tested on emacs-unicode-2, 
sorry):
M-x rmail			; enter rmail
i RMAIL.redecode		; open the file
M-x rmail-redecode-body 	; try to redecode the message in it
iso-8859-1			; as iso-8859-1

=> On my system, rmail buffer is now un-narrowed, and there
is an  "Invalid Search Bound (wrong side of point)" error showing.

Also included illustrations:
rmail_before_redecode.png is a "before" screenshot.
rmail_after_redecode.png is an "after" screenshot.