We deliver on our iteration commitments.
Imagine that the power cable for your workstation is just barely long enough to reach the wall receptacle. You can plug it in if you stretch it taut, but the slightest vibration will cause the plug to pop out of the wall and the power to go off. You’ll lose everything you were working on.
I can’t afford to have my computer losing power at the slightest provocation. My work’s too important for that. In this situation, I would move the computer closer to the outlet so that it could handle some minor bumps. (Then I would tape the cord to the floor so people couldn’t trip over it, install an uninterruptable power supply, and invest in a continuous backup server.)
Your project plans are also too important to be disrupted by the slightest provocation. Like the power cord, they need slack.
The amount of slack you need doesn’t depend on the number of problems you face. It depends on the randomness of problems. If you always experience exactly 20 hours of problems in each iteration, your velocity will automatically compensate. However, if you experience between 20 and 30 hours of problems in each iteration, your velocity will bounce up and down. You need 10 hours of slack to stabilize your velocity and to ensure that you’ll meet your commitments.
These numbers are just for illustration. Instead of measuring the number of hours you spend on problems, take advantage of velocity’s feedback loop ...