You have to pick:

This one is troubling. Cacti are made of organic compounds and water, if one is hot enough to be incandescent it would explode into vapor or at least instantly burn. If it's red hot and yet still recognizably a cactus it's not made of normal cactus material.

its a magic cactus

@brian: ... scienced.

Like, Heisenberg would approve?

This would result in a large explosion, followed by several nasty poisons if you survive that.

For starters, 11 of the elements are gasses at room temperature, and 1 gram of gas fills rather a large volume. Worst case is hydrogen, it fills 11.1 liters at standard pressure and temperature. So if that starts out crammed down in a small volume to fit inside the leg it'll expand rapidly when the force fields are turned off. The others are, in liters: Helium - 5.6; Nitrogen - 0.80; Oxygen - 0.88; Fluorine - 0.59; Neon - 0.83; Chlorine - 0.64; Argon - 0.56; Krypton - 0.27; Xenon - 0.34; Radon - 0.10

For a grand total of 21.7 liters. Google now has "volume of a human leg" in my search history, thanks. Turns out an average human leg has a volume of about 10 liters. So even if the whole leg was taken up with just the gasses, it doubles in volume when the force fields are turned off. More likely they'll have to be rather more compressed than that, so we'll get more of a bang.

Worse, we already have some highly reactive elements in the list. Hydrogen and oxygen, aka rocket fuel. Chlorine, the first poison gas used on a large scale in warfare - it forms hydrochloric acid when exposed to water. And fluorine, widely recognized as the most rabid of elements - look it up, most periodic tables will have a footnote indicating that this element is rabid and should be shot on sight (if yours doesn't it's probably defective). Fluorine will form hydrofluoric acid on exposure to water (or anything with any hydrogen at all in it - it'll rip the hydrogen out of it, and then react with the element it ripped the hydrogen off of as well). The treatment for exposure to hydrofluoric acid is amputation of the affected limb, generally speaking. So we're in a bad place even after the leg has finished popping like a balloon.

Most of the other elements on this list are pretty much inert, their only real danger comes from acting as shrapnel when the leg detonates. There's a couple of alkali and alkaline metals that are almost as rabid as Fluorine, though. Sodium, Potassium and Calcium metal will burn in contact with water, Rubidium, Strontium and Barium will burn *violently* in contact with water, and Cesium will explode like a grenade in contact with water. Check it out on Youtube, it's neat.

Last but certainly not least, Astatine (86) is on the tail end of the list. Its most stable isotope has a half life of 8.1 hours and little is known about its physical characteristics because macroscopic quantities of it have never been observed.

So if the mechanical explosion from the expanding gasses doesn't kill you, and the poisonous fluorine and chlorine doesn't kill you, and the burning, exploding alkaline and alkali metals don't kill you, you'll die from the radiation being emitted by the Astatine (and possibly cooked by the heat the decay is generating - it's expected that Astatine will remain permanently in molten form due to its own radioactive self-heating).

Oh yes, Arsenic, Cadmium and Thallium are also highly poisonous. But they're solids so they'll probably be blown away from your body before you get any in your bloodstream. Avoid dust or smoke from the burning remnants.

Actually, it occurs to me now that the physical arrangement of the element cubes might be important. On the periodic table the gasses are almost all over on the right edge. You *might* have some chance of survival if the table is oriented so that this edge is the part closest to your hip, in which case the expanding gas will blow your leg off and propel the rest of the element cubes *away* from you. You'll still be directly exposed to Fluorine, which I suspect will be enough to do you in, and Astatine is so radioactive that even that moment of exposure might be deadly. But that's the only "out" that I can think of in this situation.