Suppose a B2B software company plans a six-page launchpad on three four-week sprints, which puts launch at the end of week 12. In this plan design can't start until the copy is approved, and development can't start until the design is.
The messaging sprint needs two customer results approved by those customers' legal teams, and the approvals arrive two weeks after the sprint was due to end. Design then runs from week 7 to week 10, and development ends in week 14.
Then in week 11 the team learns the blog is moving to a new URL structure. Its 300 posts will have new addresses, and each old address needs a redirect to the new one. The developer is the only person who can do that work, and it takes a week. The site launches at the end of week 15.
The page count never changed. Two weeks came from a late input and one from a job that wasn't on the plan. Had the URL change been identified at kickoff, and had someone been free to map the redirects during the earlier sprints, that week could have stayed off the schedule.
A few questions at kickoff can help reveal dependencies like these. Who approves each page, and by when? When will the proof points and product material be ready? Which systems does the site connect to, and what moves from the old one?