It's practically common knowledge that microwaving metal objects can damage the microwave. I've heard this multiple times, mostly from my mom, and I've always believed it to be true. But is it? Would the microwave really be damaged by just microwaving a fork or an aluminum foil?
It depends on the metal and the microwave.
This video shows what happens with a crumpled sheet of aluminium foil:
Gold banding on cups is also notorious for causing arcing.
However, I was mildly surprised to learn that metal can be placed in a microwave - the USDA says that smooth aluminium foil should be okay (but make sure you read your microwave's instructions). However, you should also be watchful for arcing and stop the microwave as soon as you see it.
The danger to the microwave itself comes from the spark - there is a probability that the current from the electric arc will pass through the magnetron and damage it. Trying to find a reputable source - but it seems to be an obvious conclusion: electric arc through electric device equals damage.
Finally - and slightly anecdotally - a Scout unit I was involved with held a "mythbuster" night, and grabbed a bunch of old unwanted microwaves and tried various things like metal spoons and such. I can assure you, there were sparks.
Shield food with narrow strips of aluminum foil to prevent overcooking.
Areas that need shielding include poultry wing tips, the ends of poultry legs, and corners of square baking dishes.
Use only small amounts of aluminum foil. Larger amounts can damage your oven.
I called the 800 support number in the manual. They had nothing substantive to add--- merely: "Well, just be sure you don't use a lot of foil. Just what you'd need to keep the tips and corners from overcooking or burning."
So, including small small amounts of metal is actually recommended in some circumstances.