| a |
| a

How many vertical bars do you see above?  Three, for sure!  Now run
the following commands (that is, put the cursor where indicated below
and press "Control-X Control-E"):

(progn
  (goto-char 1)
  (set-mark-command nil)
  (forward-line 2)
  (fill-region (region-beginning) (region-end))
  )                    <-- PUT CURSOR RIGHT AFTER THIS CLOSING PARENTHESIS AND PRESS "Control-X Control-E

Now you see only *two* vertical bars!  Where did the third one go?

Tested on

GNU Emacs 23.4.1 (i686-pc-linux-gnu, GTK+ Version 2.24.21) of 2013-10-25 on akateko, modified by Debian
GNU Emacs 24.4.1 (x86_64-pc-linux-gnu, GTK+ Version 3.14.5) of 2015-03-07 on trouble, modified by Debian