In China, eggs are sold by weight and their sale is often unregulated, particularly at local markets and in areas where the customers by large quantities at a time. Unscrupulous vendors have been known to pass off fake eggs to their unsuspecting buyers.

There are three types of faked eggs. The first are empty or cracked shells that are filled with soil. Since the eggs are sold by weight, a few soil-filled eggs in a crate will make it heavier and go unnoticed until it is too late. The second type of fake egg is the red-yolk egg. Because the redder yolks are considered to be more valuable and of higher quality, some egg-sellers feed their chickens very high quantities of food dyes, which causes them to produce red yolks without an increase in the quality of the egg or in the lifestyle of the chicken.

The most disturbing - though some might say disgusting - faked egg is the man-made egg. 

The process involved in making man made eggs is fairly complex and can be seen here, but there are agents in China that run classes and sell the equipment necessary for forging the eggs. The fake eggs look almost identical to real eggs at first sight, but are made with entirely artificial ingredients. The main difference is in the way the egg performs when cooked. The "yolk" melts quickly into the "white," and the white will bubble unnaturally when fried. The eggs are tasteless and relatively harmless when consumed in small quantities, but can cause illness with prolonged or repeated exposure. The ingredients include "baifan (alum), gelatine, lactone, carboxymethyl cellulose, calcium choride, sodium alga acid, sodium benzoate, lysine, paraffin wax, calcium carbide [and] gypsum powder," among other things.

The fake eggs cost about half of what the real eggs do, so a dishonest vendor could at least double his profits by selling them.

[Thanks to David, for the tip!]