This article demonstrates why medical devices must contain free software: crackers are able to, with this particular type of pacemaker, exploit the device to trigger a fatal electric shock to its host from as far as 30 feet away (the article also mentions rewriting the firmware, which could of course be used to schedule a deadly shock at a predetermined time). These issues would not exist with free software, as the user and the community would be able to study the source code and fix any defects (or hire someone who can) before placing it in their bodies.
(Note that this article mistakenly uses the term “hacker” when they really mean “cracker”.)
The aforementioned article is an excellent supplement to a discussion on free software in pacemakers. In particular, I had pointed out within this discussion a talk by Karen Sandler of the GNOME Foundation regarding this issue at OSCON 2011, in which she mentions potential issues of proprietary software in pacemakers and the difficulty she faced in attempting to get the source code for one that she was considering for herself.
(Please do not use YouTube's proprietary video player to view the mentioned YouTube video.)