In response to the Chicken McNugget Photo Hoax, which was a photo that was released "exposing" the mechanically separated chicken that is found in Chicken Nuggets (later known as Pink Slime, see picture above).
Although this 'pink slime' does exist, McDonald's chicken nuggets, nor any of their other foods contain mechanically separated chicken.
Mcdonald's Official Statement
Hoax- Slayer, a website deticated to providing people with the full truth on internet hoaxes and spam, has a lot of reliable information to give us a full truth on Pink Slime, and what foods it may be in:
Processed foods = lots of preservatives
McDonald's keeps their ingredients public here.
There have been many 'experiments' done on McDonald's food in which they've been left out to 'rot' and have not in fact rotted. Usually, conclusions are drawn that there must be so many chemicals have been put into the foods. But the truth is, most processed foods in the grocery store that contain the same amount of salt, or low moisture content will take a very long time to decompose as well.
This article had lots of information on the everlasting ingredients of processed foods in general.