Aardvarchaeology has a discussion on some issues with radiocarbon dating. Nothing big, just interesting.
Radiocarbon dates the moment when the tissue concerned stopped receiving carbon from the environment. For a leaf, this is the moment it stopped photosynthesising. For the soft tissues of animals (who, through the food chain, receive carbon taken from the air by plants), it’s a matter of months. For bones, a bit more than that. And for dentine, the interior of teeth, it’s years or decades.
Charcoal is tricky. Every year ring in a tree has a different radiocarbon date. Chop down an old oak, sample the centre of its trunk and some of its leaves, date both samples, and you’ll get a discrepancy of centuries. This is because once a year-ring has formed, it ceases to receive carbon from the living parts of the tree. Therefore, I’ve left my selected charcoal samples to a wood anatomist. Often he can select young twigs or pieces of bark, with a low intrinsic age. Second best, he can select bits of charcoal from a tree species with a short maximum lifespan. Oaks live for centuries, but alders and aspens mostly don’t, so they’re better. Cereal grains and other seeds are excellent, no intrinsic age at all.