"Server Attention Span", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License
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They have to keep the adjacent rack units empty. Otherwise, half the entries in their /var/log/syslog are just 'SERVER BELOW TRYING TO START CONVERSATION AGAIN.' and 'WISH THEY'D STOP GIVING HIM SO MUCH COFFEE IT SPLATTERS EVERYWHERE.'