Unrecoverable data loss

[Stephen Berman]

1. emacs -Q

2. Type text in a buffer in accordance with normal Emacs usage (plain
text, code in some suitable mode, email in e.g. Gnus, etc.).  I'll call
this buffer A.
 
3. Type (in any buffer) C-u C-x =, which pops up a *Help* buffer.

4. Return to buffer A, if you're not still there, and type `M-x
insert-buffer RET *Help*', thereby inserting the output of the command
in step 3 into buffer A.

5. The inserted text retains the properties in buffer A that it has in
the *Help* buffer, in particular clickable links.  Click on the link
displaying the value of the preferred charset, or, if there are text
properties in the description, you can click on one of these links.
(Clicking the link of the value of the code point, or the button
labelled "customize what to show" does not produce the problem.)

6. Now buffer A is in Help mode and contains only the Help text of the
clicked link; the text you had previously type is gone.  If you type
C-_, Emacs beeps and says "Buffer is read-only".  If you toggle the
read-only status and type C-_ again, Emacs beeps again and says "No undo
information in this buffer".  If you were typing in a buffer that does
not have auto-save, its contents prior to step 5 is AFAICT unrecoverably
lost.

Note that if, instead of using insert-buffer, you copy the content of
the *Help* buffer either with M-w or with the mouse and then yank it
into buffer A, then the yanked text does not have clickable links.

In GNU Emacs 23.0.60.2 (i686-pc-linux-gnu, GTK+ Version 2.12.0) of
2008-05-15 on escher

Steve Berman