What happens if I plug a 120 watt appliance into a 220 watt socket?

Poof, all of the magic smoke will be let out. Depending on the appliance, it may just got poof, or it can be a bit more like the Forth of July fireworks with sparks, flame, etc.

It is not 120 watts, it is 120 VOLTS. The plugs are different so not sure you can do it physically. If you hard wire it you can burn it out immediately.

Magic smoke-that's good, one to remember.

If you are real lucky, just a fuse will blow.