As the documentation says that RET will comint-send-input anywhere in a shell mode. The issue is that if you by mistake hit enter on any line and you are not at a prompt it will execute the whole random text until the next prompt. How can I prevent this from happening? It would be nice if hitting Enter anywhere out of the prompt will send you to a new prompt at the bottom.
Something like this?
(defun my-comint-send-input-maybe ()
"Only `comint-send-input' when point is after the latest prompt.
Otherwise move to the end of the buffer."
(interactive)
(let ((proc (get-buffer-process (current-buffer))))
(if (and proc (>= (point) (marker-position (process-mark proc))))
(comint-send-input)
(goto-char (point-max)))))
(with-eval-after-load "comint"
(define-key shell-mode-map [remap comint-send-input] 'my-comint-send-input-maybe))
You could replace (goto-char (point-max)) with (comint-copy-old-input) to insert but not send the old input at the new prompt; but that's still liable to cause problems when the inserted input looks like output.
However, also note the comments and link in C-hf comint-send-input regarding comint-get-old-input -- this can be used to implement custom logic for establishing what the "old input" should be when comint-send-input is invoked with point before the process mark.