I once cut the internet for an entire school by plugging in a router.

They had a bunch of misconfigurations in their spanning tree and loop detection systems, and detected the router as a loop. That triggered the network to reconfigure itself using a different route, which was invalid, so the whole network went down.

Their IT wasn't gonna be there for a few days, and I didn't set up their network or had any access to it, so I think it stayed down for almost a week.