Maybe not the best example, but this was on FPGA. When making custom chips, you can make all sorts of tuning, including consistent hardly noticeable drifts from

Maybe not the best example, but this was on FPGA. When making custom chips, you can make all sorts of tuning, including consistent hardly noticeable drifts from spec. The article says they only image the metal layers, so I guess you can do anything as long as you don't visibly change the metal layers.