Before deploying any patch/service pack into a production environment, we test it on non- production or non critical machines first.  Desktop testing can be done by deploying on a few non-critical representative desktops.  Testing for mission critical servers can be more extensive and may include building a replica server.  These tests identify problems specific to the target environment before they become a problem.  Our test lab includes multiple physical and virtual servers that allow us to simulate different environments. 
The following kinds of tests are typically performed:

• Before and after performance tests (memory usage, boot time, disk performance, and CPU usage) on a variety of hardware.
• Test server environments with different applications installed such as SQL, Exchange, IIS, terminal Services installed.
• Test third parties services such as Novell Client, VPN, dial up clients, anti-virus software that are typically affected by Microsoft Service Packs.
• Test roll back plan in case installation fails.
• System stress tests.