Justice Secretary Keith Brown has said an IT glitch has resulted in hundreds of Scottish offenders being given an inaccurate risk level.
Risk scores are used in sentencing and prison release decisions.
But an error means the score did not change when new information was entered.
Tory MSP Jamie Greene said the “horrendous blunder” could have put the public at risk by allowing prisoners to be released when it was not safe to do so.
According to Brown, feedback from 150 users of the system had not raised any public protection issues so far.
Over 1,000 cases are being reviewed and a paper-based system is being used in the meantime.
In a statement to MSPs, Brown said a problem had been flagged up with a case management system (LS/CMI) used in criminal justice.
Offenders are given a risk score, but Brown said cases had been discovered where the score was incorrect as it had not changed when new information was entered.
There were 1,317 assessments where scores in the system “did not match” the final risk level calculated - 1,032 of them “closed” or completed cases, dating back to 2012.
In 537 of these cases, social work teams applied an over-ride, based on professional judgement - leaving 495 which “appear to contain a risk level affected by the system error”.
All the cases where an over-ride was applied will also need to be reviewed.
And the 285 open cases where errors have been found are also under review.
Greene said: “This is a grave error on the justice secretary’s watch no matter how he spins it.
“He claimed that the public have not been put at risk but the reality is he doesn’t know yet. He has absolutely no idea if this mistake has resulted in harm to the public. That is shocking and unforgivable.”
Scottish Lib Dem MSP Liam McArthur said the situation was “profoundly concerning”, adding: “Over 1,000 people could have either suffered longer sentences, or been released much sooner than they should have. There are no winners in this situation.”
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