What's the most damage a single action has done to your home?

I used a convection toaster oven in the kitchen.

That fried the circuits of 1/3 of the house. Some amateur electrician tied in several new circuits into the same kitchen breaker. They used undergauged wire on top of it.

Cost us $6k to fix and that was a hell of a deal from some Covid - unemployed union electricians. They put in 3 breakers where there was one before. The electrician had a hell of a time tracking where all the lines came from. That included a 3ft fireball shooter ng out of the wall because a line was love when it shouldn't have been.