Learned the hard way to properly label two identical bits of hardware before working on them. One ISP router in production with 20k active connections... one "

Learned the hard way to properly label two identical bits of hardware before working on them. One ISP router in production with 20k active connections... one "backup" router fresh from the box. My job was to backup the production firmware and flash the config to the spare box. The opposite happened and the customer support telephones lit up like a Christmas tree.