Six prisoners in Canada used nail clippers and other makeshift instruments to break out of prison.
While fellow inmates blocked the guards' view, they were able to dig through to an outer wall, which they then scaled to escape.
They were later recaptured and said they never expected to get away with it but the work gave them something to do.
A government report has made more than 20 recommendations to improve security at the prison in Saskatchewan.
It had been examining how the prisoners managed to escape from the Regina correctional centre last summer.
The report said investigators could accept inmates deceiving staff on a particular shift but not that "an entire corridor of inmates can deceive at least 87 corrections workers... and engineer an escape of this magnitude".
The prisoners, four of whom faced murder charges, used modified nail clippers and other tools to remove a heating grill and chip away at the brickwork until they were able to smash through the wall.
They were able to toil away at this while their friends blocked the view of the staff by playing cards in the corridor.
The six inmates escaped the compound in the prairies of central Canada by using blankets and sheets to scale the outer walls.
"We didn't think we would get away with it," one escaped prisoner told the investigating team.
"We started working on it. It was something to do and we just kept at it. When we didn't get caught, we picked our night and just went."
The Saskatchewan government has accepted the report's findings and said it will build a new C$87m ($68m; £49m) remand centre.