Not all old wood is charming. Say you're installing a new window and you come across some pressure-treated framing you'd like to replace. If it was made before they stopped injecting the lumber with arsenic as a preservative, don't touch the stuff—at least not with your bare hands. And no, it's not a good idea to cut it up and burn it in the fireplace. If the animal studies are any indication, it's not worth the risk. Though arsenic is mainly known for causing developmental problems, it also has a lengthy track record for causing miscarriages and stillbirths.
Source: "Developmental and reproductive toxicity of inorganic arsenic: animal studies and human concerns," Journal of Toxicology and Environmental Health