"Oops!" Those dumb mistakes we've all mad

theotherbastard writes: I've recently made the step up from Desktop Support to Systems Administration and in my first month on the job I made a change to a server that knocked one of our customer call centers offline for nearly 4 hours. It was the simple mistake of changing the duplex settings on 2 NIC's on 1 server. Needless to say I've learned my lesson on when and how to make even the smallest change to our sensitive systems. Another thing I've learned is that everyone on my team has a story about their first days supporting servers and the disastrous mistakes they've made. I'm curious what stories some fellow /.er's have.