[femto]

My understanding is that if you want to put metal in a microwave keep it away from the walls. If not, you can get an arc jumping from the metal to the wall and it can burn a hole in the wall of the oven.

You should also put something (eg. a cup of water) in the oven to absorb some of the microwave energy. The metal doesn't absorb much energy so without an additional absorber the microwaves can get reflected back into the magnetron and overheat the oven.

[obloid]

A while back on a whim I put my stainless steel coffee cup in the microwave and it just warmed the coffee. No sparks or drama. The metal didn't even get hot.

[nine_k]

If your piece of metal is sufficiently thick, the eddy currents will not overheat it to the point of failure. If its surface is sufficiently smooth, there will be no arches jumping off the sharp edges. Crumpled foil is usually very much unlike that, and it gets violently destroyed. A steel coffee cup with a smooth top would likely do just fine.

My microwave oven comes with a metal stand that you're supposed to put into it when doing certain kinds of baking. Likely it's engineered to be massive enough, it has rounded edges everywhere, and is supposedly sized so that there are no pieces that would resonate at 2.4 GHz and thus consume a lot of energy and overheat.