Bone rotation applied twice In this image, the top bone is rotated by 45 degrees, but the model is deformed like there was a rotation of 90 degrees. This also happens for translation and scaling.

For each vertex, the sum of the weights is one.

What am I missing? The code for skinning is inspired by the second piece of code here: Rigged mesh

Thanks
This is caused by multiple armature modifiers on the mesh. Delete one to get better results. Notice that for each number of armature modifiers on the mesh, it (the mesh) is bent again by the same angle.