Often designers do complicated things that improve the capacity on a particular hardware platform when it might actually be cheaper to buy more hardw… - Martin Fowler

When you feel the need to write a comment, first try to refactor the code so that any comment becomes superfluous.

Refactoring (noun): a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing the observable behavior of the software. To refactor (verb): to restructure software by applying a series of refactorings without changing the observable behavior of the software.

The key is to test the areas that you are most worried about going wrong. That way you get the most benefit for your testing effort. It is better to write and run incomplete tests than not to run complete tests