Landlady of the Soho Foundry Tavern Karen Clifford said Lewis Harwood paid the pub a visit just after opening on Monday last week, hours after he was let out of HMP Birmingham.
Her son Darren Clifford, who helps run the bar, went to school with Harwood.
G4S, which runs the jail, said it had correctly followed its release procedures and put the error down to the late arrival of a court fax.
Ms Clifford said: “He was in prison for something he’d done, but he was also on remand for another charge.”
She said Harwood popped in “now and again” to the pub in Smethwick in the West Midlands which is only 10 minutes walk from the jail where he was being held.
“He’s a nice lad, I’ve known him since he was a child,” added the landlady.
Ms Clifford said Harwood knew he had been wrongly released but had a bacon sandwich washed down with two bottles of Budweiser beer at the pub, before voluntarily walking back to the prison.
Harwood, of Rugeley, Staffordshire, was supposed to be let out after serving a sentence for a motoring offence.
However, he was then supposed to be kept in jail on remand for an unrelated charge.
The prison carried out its pre-release checks but it is understood a fax from Birmingham Magistrates Court with instructions to keep Harwood on remand was not sent to the jail until 5.45pm on the Friday.