Mags made the "apple pie" 'shine, and she used to share it with everyone. When she wanted to kill someone, she sat down with them and had a glass of apple pie. In order to not consume poisoned 'shine herself, she put the poison in the glass of the intended victim.
Like I said, it was subtle, and it wasn't that big of a deal. I just thought it was a cool little shoutout, since Timothy Olyphant plays Raylan Givens.