In , aemeijers wrote
in part:

Had a similar problem once. Turned out the box was cockeyed in the wall,
and they 'englished' the switch to get it as close to vertical as
possible. This was a large switch with a dimmer, so clearance was
marginal anyway. The screws holding switch to box had worked loose, and
the hot screw got close enough to the box to short out and trip breaker.
FWIU, this is a common problem when boxes are too deep in wall, and they
'float' the device on the drywall with the mounting ears, or on a stack
of washers or twist of wire, instead of using a proper extension ring.

I once saw a fixture short. The cause: The wires leading to the socket
got twisted, because the socket had turned, because someone screwed bulbs
in too tightly. The insulation on the wires got soft when the bulb was on
and making heat, then POW!

Actually, that time the short was not a dead short. Instead of the
breaker popping immediately, the fixture spewed purplish flame for about
half a minute.
--
- Don Klipstein )