Sometimes your team will work on a PBI, finish it in the Sprint, and receive feedback about changes or extensions that weren't originally asked for in the Acceptance Criteria.
Bad Example: Unlimited new tasks get added to that PBI so it takes multiple Sprints to finish
Good Example: A new PBI is added to the backlog called "<original PBI name> v2". If this then happens again, a v3 is created, and so on
This has the added bonus of allowing the Product Owner to see, at a glance, how many rounds of feedback he's given to the team on each PBI.