You have a multi-day event span (eg. 30th May to 3rd Jun), edit the event, and change the start date. The end date is also automatically changed to be the same as the start date. This will catch-out many users and causes problems.
If auto changing the end date, then it should be by plus/minus the number of days that the start date changes. Or don't auto change the end date.
If auto changing the end date, then it should be by plus/minus the number of days that the start date changes. Or don't auto change the end date.