If you move a task to Done on the sprint work board and then you move it back to another column (Review, Doing, Backlog) the burndown chart still shows the points closed today for that task i.e. once you have closed (Done) a task the burndown chart does not recognise when the task was re-opened.

This incorrect behaviour can occur when someone accidentally drops a task in the Done column and then moves it to the correct 'open' state column.