Wood is funny that way... You start out with a perfectly straight piece, mill, drill etc and all of a sudden it moves on you. Actually, I have seen the same thing happen with steel, especially when you mill it really thin. There is always some stress and tension inside the material, and when this is released you'll end up with a warped piece.
One way to get around that is to work with an oversized blank, and true it up once all the cuts are made. There's still no guarantee that it won't keep moving, but a good coat of oil usually seems to stabilize it. Denser wood species don't move as much, but pine, poplar and such can really act up.
Wonder how they did it back in the good ol' days?
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