Emacs lisp "shell-command-on-region"

Answer 1

Well, I'm not sure where the garbage is coming from, but the error itself is coming from shell-command-region. When used in elisp, it expects at least 3 arguments, START END and COMMAND.

Also, in general, it is bad practice to mess with the mark in functions. Here is what the doc of push-mark has to say on the subject:

Novice Emacs Lisp programmers often try to use the mark for the wrong purposes. See the documentation of `set-mark' for more information.

Answer 2

I'm unsure what you're trying to accomplish with the pushing and popping of the marks, I believe you'd get the same functionality by doing this:

(defun figlet-region (&optional b e) 
  (interactive "r")
  (shell-command-on-region b e "figlet")
  (comment-region b e))

The argument to interactive tells Emacs to pass the region (point and mark) in as the first two arguments to the command.