@CoinageJohn: Your image does not provide enough of a close-up view for me to give a definitive answer regarding what has happened with your coin but I'll offer a couple of possibilities.
Is the "hair" raised on the surface of the coin or is recessed into it? If it is a raised line, it is possible that the coin was struck with a scratched die. In this case, the scratch in the die would be filled with silver when the blank planchet was struck by the die.
It is also possible that you have what is called a "retained struck-through" error. This type of error involves a piece of debris (in your case, a small piece of thread is more likely than a hair) that finds its way into the striking chamber and is pressed into the surface of the coin when it is struck; a portion of the material is retained in the planchet after striking.
Without better pictures of the area in question, I can't say much more with any certainty.
While error coin collecting is very popular among circulation issues, such minor errors as what appears to be shown on your coin are not collected as avidly. As a result, selling the coin at much of a premium might prove difficult.
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I wouldn't go so far a to call it a mule.
It seems as if you are confused about what a "mule" coin is. The term "mule" is used to describe a coin that has obverse and reverse designs that are not supposed to be paired together. One of the most popular mules is New Zealand's 1967 two-cent coin. It was struck with the correct New Zealand reverse die, but featured an obverse struck with a die intended for a Bahamas coin. Two popular mule coins from Canada are the September and November 1999 versions of the commemorative "Millennium" 25-cent coins. They were each struck with an obverse die that was supposed to be paired with a die featuring the standard Caribou reverse design; as a result, the coins do not include the "25 Cents" denomination.
Here's a link to an informative page on Canadian mules:
http://www.coinscan.com/err/mmd.htmlHope this all helps! Additional questions are welcome.
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