After some discussion, we concluded that the horrendous weather in the eastern United States could be responsible. As it turned out, that wasn't the cause. Instead, an unexpected data condition caused the data analysis routines to misbehave. I should have known better. Check the raw data, check intermediate calculations, check the algorithms.
Chances are, something went wrong along the way.
Computers rarely suffer internal processing failures that they don't catch. Well-written programs rarely fail to calculate properly. But calculations are no better than the algorithms used to code them, and algorithms are no better than the designs used to prepare them. Spreadsheets and other similar programs may represent the worst case. With most spreadsheet software, there's nothing at all to prevent a user from keying a number into a slot that should be a calculation, thus disrupting not only that particular cell but any other cells that depend on it.