A flip of a coin does not result in an exact 50-50 chance of heads or tails. It depends a lot on how the coin is flipped, of course, but there is a bias. This article explains when, why, and how much bias there is.
Doug Steigerwald. The Coin Flip: A Fundamentally Unfair Proposition. Available in html format
You can find an earlier version of this page on my blog.