There's been tales of a 12 year old McDonald's burger passed along the internetz--and of course, I'm willing to believe the worst of McDonalds (aren't we all).
But J. Kenjii Lopez-Alt, being a scientist, wanted to test whether this was true --
Thus far, I haven't located a single source that treats this McDonald's hamburger phenomenon in this fashion. Instead, most rely on speculation, specious reasoning, and downright obtuseness to arrive at the conclusion that a McDonald's burger "is a chemical food[, with] absolutely no nutrition."
--and did so, brilliantly here--
http://aht.seriouseats.com/archives/2010/11/the-burger-lab-revisiting-the-myth-of-the-12-year-old-burger-testing-results.html
His results are pretty interesting--I won't give them away, but, just a warning--don't eat the Big Mac you just found in a pocket--the plain burger's ok, though if you're desperate.....
(thanks to CanadianChick for this)