Starvation mode.

It's complicated.

It's not likely that you would gain weight in 'starvation mode'.

I like this blog post: http://fattyfightsback.blogspot.com/2009/03/mtyhbusters-starvation-mode.html
"Mostly because, the more you reduce your intake, the harder it is to get the nutritients you need from food. If you are on a very low calorie diet (as I am), you need to see your doctor(s) regularly, get labs done regularly, etc. Not to mention, vitamin supplementation is a must. Doing what I'm doing on your own can be dangerous, as you may not know or noticed the signs of a vitamin or mineral deficiency. Don't forget: some vitamin deficiencies can kill you!"
"In the end, it's important to consume enough calories that you have the energy to perform the daily activities you want to and to keep your body healthy. Otherwise, it's self-defeating. After all, the point of losing weight is to be healthier and to get our lives back. It's not to starve ourselves to the point of malnutrition and have so little energy we can't go out and do fun things."

Don't pay too much attention to the comments - but DO read the articles the blogger linked to!

Starvation mode is bad and you don't want it. It's supposedly an evolutionary thing that happened to help us stave off famine. What happens is that when you eat very little, and from what I've read that's usually less than 1000 calories, your body shifts into this mode. Normally when you eat, your body uses the calories stored in sugar (glucose) and body fat to sustain itself. However, if you're starving yourself to the point of famine, your body will have used up all those calories already. So what does it do? It turns to the lean tissues and muscles in your body and starts to suck away some of the calories from them (from their proteins & amino acids).

When you lose muscles & tissue, that's very bad. You might lose weight while doing it, but it's not the good weight. You want your muscles and tissue. Real bad. In fact, if you starve yourself, lose muscle, then try to go to back to normal mode, you'll be worse off. When you have less muscles that means your metabolism is lower -- basically you're less muscular now, so your body doesn't need as much calories to keep going. Guess what? When your metabolic rate is lower, your body starts to do much worse at losing weight.

Does any of that make sense? I'm totally not a scientist here, and some of this may be not 100% accurate, but someone else may chime in. Starvation mode is definitely not a good thing. You'll destroy your muscles and slow down your weight loss. That's about all I know.