Working on a Team Foundation Service 2012 project. The iteration start and end dates created in the system were (are) 14th May to the 6th of June, a 4 week sprint. On the 13th of May we had our review meeting and created all of the tasks in the application and the total commitment for the iteration was 318 hours. This was shown on the home screen and the burn down chart correctly at the end of the 13th of May and the actual start of the iteration on morning of the 14th of May.
However, on the 14th of May when tasks where being completed the starting point of the burn down was coming down rather than staying at 318 and the bar moving down in a downward fashion. Only on the second day this started working correctly however from the burndown it looks like we actually started on 285 hours rather than 318 looking like we actually commited to the 285 hours rather then the 318.
In our first iteration (16th April to the 9th May) the burnchart looked correct as it started with the bar chart coming out to display the first days work.