UPDATE: We experimented with other burgers to break the myth of the 12-Year Old McDonald's Burger That Just Won't Rot.
The McDonald's hamburger on the right is from 2008; the one on the left is from 1996. And they both look fairly edible.
Wellness educator and nutrition consultant Karen Hanrahan has kept a McDonald's hamburger since 1996 to illustrate its nonexistent ability to decay. Aside from drying out and bit and having "the oddest smell," it apparently hasn't changed much in the past 12 years.
This isn't the first time someone kept an uneaten McDonald's hamburger for an extended period of time for the sake of science. Or in the case of the Bionic Burger Museum, multiple burgers for over 19 years. There are even instructions on how to start your own collection of old, self-preserving burgers.
Anyone else have experience with Fast Food That Just Won't Rot?