At the first office I worked at, we had 2 different ISPs. This was supposed to be for reliability, as one was fast but spotty, and the other was slow but reliable. Since they weren’t too expensive, we just went and got both.
We have monitoring setup to watch our office IPs from the outside so we could see how often the connection goes down. The interesting thing we found was that the fast and spotty connection had perfect uptime. Even when there was clearly no internet from the office, it was still “up” according to our monitoring.
So we tried pinging our office IP using the other connection and to our surprise it was indeed up. There was even a webserver running on it (we only have VPN exposed). Apparently, it was someone elses CCTV admin page. We could actually see a hallway with people walking by sometimes!
Apparently someone else had our IP address and nothing good comes from an IP conflict. This was completely baffling as our internet line was supposed to be a “business line” and that came with a static IP address. So the only scenarios where this could happen is, the ISP mistakenly gave the same IP to 2 different lines or the ISP allows some clients to freely set their own IP.
We complained to the ISP and eventually got it resolved. They just gave us an entirely new IP address, but they never explained what went wrong. We already had quite a negative opinion of that particular ISP though, and they somehow managed to outdo themselves.