Final answer:
Items in the product backlog do not need to be well defined with acceptance criteria at all times. Their detail level should correspond with their priority and proximity to implementation.
Step-by-step explanation:
The statement that 'at any given time, all items in a product backlog must be well defined and have acceptance criteria' is false. The product backlog is a dynamic list of features, enhancements, and bug fixes that is prioritized based on business or customer value. It is a living document in which items are continuously evolved and elaborated upon. Early in a project, backlog items may be broadly defined or represented by epics. As the project progresses, these items are split and detailed into user stories with clear acceptance criteria when they are closer to being worked on. The level of detail and clear acceptance criteria for each item in the backlog should be proportional to its priority and the time horizon of its planned implementation.