Just thought I'd post this -
During ''The Ultimate Heist'', the final Thieves' Guild quest, it is possible to keep the Boots of Springheel Jak, though it's pretty high recommended you have a high Sneak skill/high Chameleon or both, since you don't want to get caught.
When you're in the palace, in the office where the trapdoor is you need to go through (and fall a long way down), there's a few bottles to be found there which will fortify your health. Drink all those and as much of the strongest health potions possible. Now enter the trapdoor WITHOUT wearing the Boots
When you land you will loose a lot of health, so heal yourself, but you will still have the Boots. And who doesn't likes 50 extra points for Acrobatics?
This can not be done always though! so far, I found the minimum amount of health required (before fortifying your health) has to be at least 120. In my case, when I jumped down I had about 10-20 health points left