Textualmente del libro:

“As with any numerical code, debugging can be the most difficult part of creating a successful program. For multigrid, this situation is exacerbated in two ways. First, the interaction between the various multigrid components are very subtle, and it can be difficult to determine which part of a code is defective. Even more insidious is the fact that an incorrectly implemented multigrid code can perform quite well -somentimes better than other solution methods!”

Puedo confirmar, de primera mano, que ésto es así…