> * ./src/emacs -Q
> * In *scratch* at least twice:
>   * Insert "aaa"
>   * Undo
> * Select the entire buffer
> * Undo in region with prefix-arg 4
> * Observe the wrongful deletion of the string: "fer." in the Lisp
>   comment.

A more concise recipe:

  * ./src/emacs -Q
  * In *scratch* insert "aaa"
  * Undo
  * Select the entire buffer
  * Undo in region
  * Observe the wrongful deletion of the period in the Lisp comment.