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The California Department of Corrections blames a clerical error in the Sara Jane Olson case for miscalculating her release date by one year early.
Sentencing experts aren't surprised that Olson is back in jail Monday night; many say it actually happens more frequently than people think. Usually, though, prison time calculations result in too much time behind bars.
"Among the cases that are reversed, criminal cases that are reversed on appeal, almost 25 percent of them are reversed because there have been sentencing errors," said Professor Michael Vitiello, University of the Pacific, McGeorge School of Law.
The State Corrections Department has to wade through numerous laws to get sentencing for each crime just right.
Calculating prison time is so complicated, that's all dozens of staffers do full time for roughly 170,000 inmates.
"Our case managers, that's what they're focused on. They do literally half a million calculations a year in terms of sentences," said Alberto Roldan, Chief Deputy General Counsel for the California Department of Corrections and Rehabilitation
The Legislature often adds enhancements for certain types of convictions, extending prison terms for crimes that, for instance, involve drive-by shootings.
Sometimes citizens pass their own laws through ballot initiatives, like Three Strikes or Jessica's Law for sex offenders, further burdening the process.
Then inmates can earn good time credit, reducing their sentences.
"All of those things affect what started as one sentence may finish as another sentence. It's very difficult," said Roldan.
"We've had so many changes in the criminal sentencing laws over time that the system is a mess," said Vitiello.