Here’s what happens to a really old wall switch:
A closeup of that broken contact:
This switch controlled an outlet, so I’m sure it’s hot-switched far too many vacuum cleaners, clothes irons, and suchlike over the last half century or so.
Our house is a bit fancier and originally had top-of-the-line mercury-wetted switches: the contacts sealed in the capsule don’t burn, but the springy supporting structures outside the capsule eventually wear out.
They’re still more reliable than X10 switches, though.
The Smell of Molten Projects in the Morning 