Hundreds of prisoners have gone on the run following temporary release, it has emerged.
The Northern Ireland Prison Service also revealed a number had been let out too early, while others were kept too long and won a total of £50,000 in compensation as a result.
Data compiled by The Detail website showed there were more than 200 incidents of prisoners failing to return, with some remaining at large for years.
The Prison Service insisted that the number was small compared to "the majority of prisoners that abide by the rules".
Since 2007 there have been 218 incidents of prisoners failing to return after leaving for town visits, home leave and more.
There were 95 instances of prisoners going missing from the medium-security Magilligan jail, and 93 from high-security Maghaberry. Thirty went missing from Hydebank, which holds women and young offenders.
The majority of inmates went missing on home leave, mostly for a few days, but one from Maghaberry went on the run for four years in 2009. All the people in the data were returned to jail.
Further figures obtained by The Detail showed that, since 2010, four prisoners had been accidentally released early from Maghaberry. Reasons for this ranged from misinterpretation of court results to a sentence calculation error. There were also more than 20 instances of prisoners being held beyond their release date during the past 10 years, with the reasons including incorrect calculation of remand time and incorrect warrants issued by the Northern Ireland Courts and Tribunal Service.
A spokesperson for the organisation said: "In October 2011 the NI Courts and Tribunal Service introduced a series of additional controls and independent verifications to strengthen the procedures surrounding the accuracy of court orders.
"In addition, it invested in the development of systems and interfaces with other criminal justice agencies and an extensive programme of staff training.
"The accuracy rate for criminal orders from April to September 2015 is currently 99.4% and is subject to close scrutiny."
amount paid to those jailed for longer than they should have been