Create two issues with estimates of 1 hour each. Spend 30 minutes on one and 90 minutes on the other.
Now resolve and close both (this should imply that work is finished, right?).
The Time Tracking report now will show that we're 30 minutes behind the estimated schedule.

Workaround: when setting the "remaining estimate" to 0 for the "good" issue, the gain will be displayed on the report.