13. Failing to virtualize
If you aren't taking advantage of virtualization, you're only making things harder on yourself. Stacking multiple virtual machines onto a single physical machine drives up system utilization, giving you a greater return on your hardware investments. Virtualization also allows you to easily provision and de-provision new systems, and to create secure sandbox environments for testing new software and OS configurations.
Some vendors may tell you that their products can't be installed in a virtualized environment. If that's the case, tell them bye-bye. This is one technology that's too good to pass up.