> • ./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.