We've seen a few tickets show up in an Active sprint, in the Done column, that we had not added to the sprint.
In the Sprint Report, they are listed in the "Issues completed outside of this sprint" section.
The issues had previously been moved to status = Done, and Resolution was set to "Won't Do".
When we look through the History of these tickets, there is no step listed of the ticket being added to the sprint -- no edit is shown.
These tickets sat in the project in that Done state for some time, before showing up in the active sprint.
EDITED TO ADD: The tickets were added to a sprint, and then that sprint was deleted.