Trees, in many ways, are like humans. Like us, they live a unique life with their environments shaping them and making them into whatever tree they become. Some trees deal with more environmental stress, like winds or extreme temperatures. This causes that particular tree to retain more internal stress than the normal amount for a tree, in some cases.
Many of these tress come from areas where storms are the norm, or perhaps the tree grew on the side of a hill. Often times, trees can develop internal stress as a result of competing for sunlight with other trees and plants. Trees growing on a severe incline result in some of the worst types of wood because the gravity forces a curve in the trunk. You have probably seen a tree like this and not even realized it.
Take trees by the ocean, for example. Many of them are subjected to constant winds, which cause the tree to have gnarled branches and twisted trunks, a definite indicator of high internal stress. If you note that a tree has a bow in the trunk, the reaction wood will lie within the area of the curve.
So what does this mean for the woodworker or craftsman? It is probably your best bet to stay away from any kind of wood floor with high levels of internal stress. Sometimes, the stress within is so intense that you can actually notice it in the new flooring discount box or from a low quality flooring company. When this happens, the separated wood starts to close in on the blade or curves away and pinches the blade. If you notice this happening, you should probably play it safe and go look for a new piece of wood floor for your project.
Avoid any wood that has abnormally curved or spiraling logs and boards. Also, an off-center pith in the trunk is a definite red flag, as well as a furry surface on the board. This fuzziness is caused by compression or tension, which is exactly what you want to avoid. It will be unpredictable, and it will be difficult to machine, sand, or finish the piece.