症状

The burndown chart for your Sprint is suddenly showing negative value for the remaining value.

原因

This problem can be caused in 2 conditions : 

1. By changing the original estimate of the issues after starting the Sprint : When you start a Sprint, the Burndown chart takes into account the original estimate for the issues in the Sprint. This number will be used to plot the burndown chart and is not adjusted if you change the estimate after starting a Sprint. So if you start a Sprint with for example, 10 hours and then start logging time on your issues. At some point you change the estimate to 5 hours, while you have already logged 6 hours to the issue, the remaining estimate will show up as negative on the burndown chart.
2. By not selecting "Adjust Automatically" while logging work but actually manually changing the remaining estimate : When you log work on an issue, you can select one of the following options : 

So if you do not select 'Adjust automatically' and choose to adjust the remaining estimate manually, you can get into a situation with negative remaining estimate as well. For example, you choose to manually set remaining estimate to 0 and then log another 1h after that. This would lead a negative remaining estimate situation. 

回避策

Unfortunately there is no workaround for this as this is expected behavior from the system.