My organization is transitioning from a waterfall style of running software projects to Scrum. Everyone involved in the project has had Scrum training, they understand the benefits, and they are on-board with the changes. Sometimes that's not enough.
The Product Owner and some of the Team were with the organization when they ran waterfall-style projects. They are accustomed to spending a large chunk of time gathering requirements and another large chunk of time doing architecture and design before beginning development. They would also set aside a large chunk of time for testing after development that they weren't able to focus on because development took longer than estimated and requirements changed mid-development. They experienced all the typical problems associated with waterfall projects.
But, old habits are hard to break. If the Product Owner is accustomed to waterfall, they they will likely want to to spend a lot of time on requirements or when discussing a feature they want to detail all the items related to that feature.
It's very important to start with epic user stories and break them down into themes and more detailed user stories as needed. When initially building the product backlog the stories only need enough detail so both the business and the team understand the functionality and they can be assigned story points. Detailing all the user stories on the product backlog when it is being initially populated can cause several problems. First, they are likely to change by the time they are implemented anyway. Second, it takes too much time to detail all the user stories in the meantime there is no value being delivered. Also, If you spend to much time delving into individual stories, other stories may be overlooked. Delay the details of the stories until just prior to implementation. Otherwise you'll slip back into the old habits of waterfall.
To break the old habits, you need a disciplined, patient, resilient ScrumMaster to help the Product Owner and the team delay the details as long as possible and to break down larger user stories into smaller ones and focus on each story individually.
Have you ever been on a scrum project where the Product Owner was accustomed to waterfall? Were their old habits hard to break?