SLA calculation in IT-Services might become a powerful feature.
Beside some minor limitations which actually won't prevent using it seriously, there is one thing that is crucial -at least in my opinion.

If SLA calculation is taken to represent the real service levels and shall be used for reporting and potentially even used for penalty, then there has to be a way to subsequently add corrections for defined time periods.

Such a correction could be defined on trigger level with parameter like 'Trigger status', 'Time period(s)' and 'Comment/Reason'.
For the case of an error that has extensive impact of SLA calculation, it would also be nice to have the possibility to recursively add a SLA-/Trigger-correction on higher levels as well.

These corrections have to be audited and be visible.
For non-graph views this could be a visual indicator with link/tool-tip to/about the corresponding corrections.
In graphs this could be noticeable by different colors.

A correction may not change the data a SLA is based of, it rather should be a separate definition that gets taken into account for on-demand calculation and displaying of SLA.

By this feature it would be possible to correct incorrect SLA calculations based on missing data or any kind of malfunctions in the processes of data collection.