In the embedded signing iFrame, there is an image included at the bottom of the interface:
https://www.docusign.net/Member/Images/logo_poweredby_blue@2x.png
I'm guessing there's a 2x image script for Retina displays, but that image does not exist and is showing as a broken image:
This is less of a question and more of a report. Users using Retina Macbook Pro laptops are seeing a broken image at the bottom of the contract.