902
xkcd #2929: Good and Bad Ideas
(imgs.xkcd.com)
Alt text:
While it seemed like a fun prank at the time, I realize my prank fire extinguishers full of leaded gasoline were a mistake.
Obviously you shut off the main breaker so as not to backfeed. Didn't think anyone would be pedantic enough to make me say it. I wasn't writing out a tutorial.
Point being, there exists a purpose for the allegedly purposeless thing.