On pages where a form needs to be filled in, the persistent navigation can be a waste distraction. For instance, when you’re paying for your purchases on an e-commerce site you want only to finish filling in the forms. The same when you’re registering, giving feedback, or checking off personalization preferences.
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.