The randomness in a coin toss, it appears, is introduced by sloppy humans.
—from NPR story linked below
A few relevant links on the topic of tossing coins:
• A random coin-toss generator.
• An article in Science News about the bias inherent in coins.
• An NPR story on the bias of the coin. Excerpt:
During World War II, South African mathematician John Kerrich carried out 10,000 coin tosses while interned in a German prison camp. However, he didn’t record which side the coin started on, so he couldn’t have discovered the kind of bias the new analysis brings out.
• Persi Diaconis’ website.
• The most complete discussion of the flipping coins that you will find anywhere.
• The ever reliable Stephen Jay Gould weighing in
on a certain toss-up of yore.