Humidity is a general important part of caring for our T's, but so is adequate hydration. Remember our T's use their body fluids/blood (hemolymph) to move and moult ...
Our T's use their muscles to move their legs inward, but they dont have muscles to extend them back out again.
They have to force their bodily fluids(mainly blood) into their legs to make them move outward (think of hydrolic pressure). A dehydrated T can't move their legs back out, and that is why we see them typically "curled up" with their legs beneath them. When we put them in ICU and hydrate them again they can extend their legs and move again
In moulting our T's secrete a fluid between the old and the new skin, allowing the new skin (exoskeleton) to form. When its time to moult they actually absorb the fluid back into their bodies leaving a "gap" between the old and the new skin which is what allows it to "seperate" during moulting.
To get out of the old skin and moult, it has to do a "jailbreak"

It literally bursts out of its seams! It pushes up its heart rate to pump a lot of their blood (hemolymph)from their abdomen into their first body "segment"(cephalothorax) This pressure expands the old skin (exoskeleton) until it cracks - then its merely a case of our T's doing a bit of pumping iron to slip out of their old skin
I hope this explains why hydration BEFORE a moult is so important! I personally dont believe that merely spraying the tank while the T is moulting is going to help any if it was previously dehydrated - note this is my personal opinion!!
Having said that - some T's just seem to have a natural tendency to moult strangely. I have a P Cancerides that was stuck/lost a limb during each and every moult ...since it was about 1cm big :blink: i was half expecting it to die with every moult. It has finally "outgrown" that problem now and at 6cm is moulting perfectly.
Some stuck moult has never bothered my T's - unless it inteferes with their feeding. I would definately try what cascade said and moisten the piece of stuck skin gently.

Let us know how it goes!
ps: sorry if my 2c worth just became 5c