Body adaptions to calorie deficit - How much do those bad days or weeks really hold you back?

I have talked a lot about balance in the past, but how important is it in a fat loss phase to focus on being as close as possible to the plan for certain periods?

When a new calorie deficit is created, the body will begin to reduce its calorie maintenance level towards the calorific intake.  This is a great tool for us to avoid plateux's by reducing the calories at the right time but we only have so many opportunities to make those cuts.  This is why it is not simply a case of "just dieting for longer"