During 4.08 Wishful Thinking, Sam and Dean discover that wishes made at the wishing well in Lucky Chin's restaurant in Concrete, Washington, come true. They drain the wishing well and find a very unusual and ancient coin at the bottom. Try as they may, they cannot remove it. They eventually discover that Wes Mondale inherited the coin from his grandfather, who found it in North Africa, but warned him not to use it.
The brothers' research suggests that the serpent on the face of the coin is the Babylonian god of primordial chaos, Tiamat. On the other side is the symbol of the Babylonian god Shamash, bringer of light and justice.
It was created by Tiamat's priests in ancient times. Using incredible dark magics, they forged the coin to sow the seeds of chaos, in the name of Tiamat.
A person who tosses the coin in effectively "turns on the well." It grants any wish made on the well. Once the wish is granted it begins to warp until the wisher no longer wants it. The wishes turn bad or "backfire."
For example, Dean wishes for a foot-long Italian sandwich with jalapenos. After enjoying his sandwich he discovers it gave him food poisoning. Also, an adolescent boy wishes to be invisible so he can spy on the women's locker room at a local gym. However, his invisibility also causes him to be hit by a car. Wes Mondale wishes for the woman he was infatuated with to fall in love with him, but she becomes a mindless, subservient worshiper rather than a girlfriend.
Breaking the Curse
The coin can only be removed from the well by the person who tossed it in in the first place. Once it is removed, however, all wishes will be undone. Sam and Dean force Wes to remove the coin, and they later melt it down to prevent it from ever being used again.