Years ago my boss was working on a partially gutted row home alone, it was the home of friend of his wife and he was just doing a favor for this person, there were some old circuits and some new and he was tracing out and testing, like any small job or 'side job' can be, he had a jbox open in the attic, and like most attics there was lots of stuff like old papers.
He reset a 20A breaker in the basement and some sparks must have fell out of a attic jbox he had opened and started a fire, He said by the time he went up to the 3rd floor he herd the crackling sound and a woosh sound. He tried to put it out, but nobody else was home and the attic quickly filled with smoke, he ended up in the hospital for smoke inhalation. Some pets perished in the fire, but nobody else got hurt, the FD damaged the neighboring houses roof. The damages were $$$, it was a catastrophic incident from something as simple as energizing a 20A breaker working late alone. He of course is fully insured and the homeowners insurance sued his insurance and his insurance settled they way that they do.
He did not have to pay one dollar. He had such a long track record of no claims (probably 30 years) his insurance barely went up.
After I saw that I never did a side job again.
I made an arrangement to run friends and family small jobs thru the company, so god forbid if anything like that happens I am covered.