Currently, UD walk's seem to happen at any random time. Which for web app's that are on a single instance can mean downtime during the cold boot phase. These should really be applied out of hours/early morning (time zone dependant on your service plan location) to avoid major disruption. In a particular instance, we had 3 UD walk's within 2 days during normal business hours. I know this can be avoided by scaling the instances but really shouldn't have to be a requirement.