If you modify a normal in the vertex shader but don't use it in the fragment shader, shadows are broken if you have any normal bias (which is the default). This is due to the shader generator not writing the modified normal world space position back to the vertex structure, which causes it to use the original, unmodified normal for shadow calculations. This breaks almost all non-trivial vertex animation.

How to reproduce:
1. Open the attached project's Scene labeled "SampleScene"
2. Enter the Play Mode
3. In Game View, inspect the "Cube" GameObject's shadow as it spins

Expected result: The shadow looks the same as when rotating the GameObject without using Shaders
Actual result: There are seams in the GameObject's shadow as it rotates

Reproducible with: 10.6.0 (2020.3.16f1), 11.0.0 (2021.1.17f1), 12.0.0 (2021.2.0b5, 2022.1.0a4)
Couldn't test with: 7.7.1 (2019.4.29f1) (Vertex shader in ShaderGraph not available)