No matter the size of your IT infrastructure, you've built in some form of redundancy, whether it's as simple as RAID in direct-attached storage on your servers or as complex as multiple, cascaded, geographically separated hot sites.
In the past, I've strongly advocated setting aside a portion of planned downtime windows to test that redundancy -- which just about everyone can and should do. But how would you feel about testing that redundancy smack dab in the middle of a production day? When was the last time you yanked a disk out of a RAID set or unplugged a redundant network link just to see what would happen?
If the answer is never -- why? After all, you've invested the capital in providing the redundancy. What does it say about that investment if you're unwilling to test it when it matters most?