Around 20 years ago, between better jobs, I accepted a job with a state government agency to design and rebuild its website. I had the only Macintosh in the entire department, which consisted of, maybe, a thousand people. Their network was some kind of old Novell thing.
Needing Internet access, I plugged my Mac into the Ethernet port in the wall and changed the settings on my Mac to match. It worked, and I went to lunch.
When I got back from lunch, there were about a dozen people standing around my Mac staring at it. Somehow or another, that Mac was hijacking the entire network and insisting that all traffic on the network heading to or from the Internet travel through that Macintosh as a DNS gateway.
Over a period of about 40 minutes, the entire agency’s network gradually ground to a halt as the Mac propagated some kind of network instruction that rewrote previous network settings. Since all the state agency networks were interconnected, the hijack started spilling over into other state agencies.
The IT people had no idea what to do since they had no experience dealing with Macintoshes. They finally shut down the entire network across the several agencies that were affected, then booted up everything from scratch and had everyone restart their computers.
They wouldn’t let me plug the Mac into the network after that, so I needed to use a Windows machine for the remaining several weeks I was there.