I was wandering around Server Fault this evening and found someone looking for stories of the Best sysadmin accident. Here's mine:

Ten, plus years ago I was working on a project that required a SOCKS proxy. I had been using a program called WinGate that in addition to SOCKS proxy, provided a nice little Internet gateway functionality with NAT, DHCP and a few other niceties. This was before Windows had Internet Connection sharing, so WinGate let you share your dial-up modem with your Ethernet network.

I installed the software and started work on the SOCKS client functionality. Later that day, we lost internet connectivity. All of a sudden, it just stopped and nobody could access outside the company. We called our ISP and everything looked fine on the connection. The router was working fine. We just couldn't figure out what went wrong. I pitched in at one point as I had some knowledge of TCP/IP, but I didn't make any headway.

The next day our IT guy figured out that the DHCP server had given the address of the router out to someone's machine, and everyone was using it for the default gateway which didn't go anywhere. Later that day our IT guy came into my office and I asked, "So did you figure out who gave out the wrong IP address?" He said, "Yeah, it's you!"

WinGate had defaulted to running a DHCP server and had given out the router address to the first client whose previous address had expired. I was pretty red-faced for a week.