Dietary fat is chemically a triglyceride. 3 fatty acids bound to a glycerol molecule. Glycerol can actually be turned into glucose and my guess is that much of the dietary glycerol would be turned into glucose, possibly providing a fair amount. But I think you would probably catabolize your muscles to a degree.

It would be a downward spiral to say the least.

The Perfect Health Diet (perfecthealthdiet.com) advocates ~70% fat. The Optimal Diet recommends ~80% fat. PaNu for weight loss recommends ~80% fat as well, and for healthy people who don't need to lose weight the PaNu fat macro is still high at around ~65-75% (though he doesn't stress particular macro ratios, if you're doing it right, it'll come out to those percentages anyhow).

The only diets that are 90%+ fat are, as you've noted, therapeutic ketogenic diets and 'fat fasts' where consumption is completely fats, and in those cases it is a last resort. If you're not experiencing terrible suffering or life-threatening conditions then why would you event want to go above 80%? Perfecthealthdiet.com has a series on risks of prolonged diets without adequate starches/carbs, and PaNu has recently come out in favor of starches up to 20% (depending on activity level) as very healthy.

It is really easy to stay in the above recommended ranges--have your meals just be meat/fish/eggs with a side of veggies/tubers all drowned in butter/coconut oil. Very simple, very healthy, and best of all, very tasty!

Death from the lack of essential amino-acids.

Diarrhea would be the first side-effect.

I'm wondering about this myself because I lost a lot of weight my first week of paleo but so far this week I've gained two pounds. I've been super super strict, unlike my boyfriend who still is eating sandwiches and protein shakes (on paleo, he says).

I'm desperate. I'm going to do a day where all I do is coconut milk or avocados and see what happens.

Maybe you're gaining muscle which weighs more while loosing fat.