Some decades ago, friends of mine ran an ISP that resold bandwidth from another business. They used a leased line to the business, and every Friday evening at about 7.30 their connection dropped out.

It turned out that the cleaners at the remote end would plug their vacuum cleaner into the same circuit that was powering my friends' hardware. The power surge caused their hardware to crash, and nobody had access to properly reboot it until Monday 8.30am.