I once lost 12 lbs in one week. Maybe 20 years ago. I'm sure it was mostly water, but it was water I was carrying around.
The problem with seeing these amazing numbers, is it can inspire crazy thinking, which can inspire crazy action.
The first few weeks of a weight loss attempt, or even occasionally a week or two in the midst of one, can throw us a small miracle on the scale, but it's the long haul that is really important.
Losing weight for the long haul is about moderation. Eating as much as you can and still lose weight at a moderate rate. Depriving yourself more in order to see larger results, can backfire quickly. But it isn't one week with a large loss that makes a plan unhealthy, it's the much bigger picture. Are you eating enough to be reasonably satisfied? Are you finding yourself cutting calories to uncomfortable levels in order to see a "bigger" loss? Are you going to want to QUIT if the weight slows to a more typical level?