I'm surprised this hasn't been reported before as I've seen coworkers gripe about this.

When using before-save-hook to run elixir-format, like so:

(add-hook 'before-save-hook 'elixir-format nil 'local)

If the formatting fails, an <original_name>-emacs-elixir-format.ex file is left in the same directory as the file for which formatting was attempted. For whatever reason, this does not appear to happen when elixir-format is invoked manually.