It is very common for software users to categorize every single unpleasant experience as a Defect.
With the same logic, it is also very common in Scrum project, that a Product Owner classified every things deem "missing" from a "Done" product as Defect.
For me, it is important that a Scrum Master, or an Agile coach to make sure this expectation is set during the project kick off. Recommend the usage of Extreme Programming method of user stories to capture requirements, emphasis on the important of acceptance criteria in the user stories. Make it a point that, any scenario that is not being thought of as acceptance criteria will be classified as enhancement. They will be added into the Product Backlog for prioritization and to be work upon. System behavior that does not match the acceptance criteria will then be classified as a Defect for the team to work on them as soon as possible.
The development team hold the responsibility to make sure all acceptance criteria in the user stories are handled. While the main purpose of the UAT is to discover more new scenario that the system needs to handle and add them as enhancement into the Product Backlog.
Avoid the situation where the development are encourage to make their own assumption and code them into the project, as there is high chance that those will be a waste to the project. All assumptions are to be clarified with the Product Owner before any single line of code is written on them.
Everyone in the Scrum Team should be encouraged to contribute to the acceptance criteria all the time. It will just make the product better.
