Back in September I issued a pull request to the Processing GitHub repository. This pull request covered an issue with a confusing error message that caused unused PShader uniforms to be removed from the compiled shader.
This was largely an issue for shader developers because if they changed the shader code that they were writing to not use a specific uniform then it would be removed from the compiled shader code, and then the Processing code that was using the shader, would throw an error when the uniform that was "missing" would be assigned to.