Street killer Nicola Edgington walked out of a mental health unit to buy a knife at a supermarket before she attacked two women.
She had been taken to Oxleas House in Woolwich, London, but walked out soon afterwards and later complained that staff kept tapping on a computer.
Judge Brian Barker, who sentenced her to life, said: "It is unfortunate that the shift was changing and one of the doors was faulty."
Edgington then took a bus to Bexleyheath, in London, and bought a knife from an Asda supermarket.
A psychiatric patient has been jailed for life with a minimum term of 37 years for killing a grandmother in a street knife attack.
A woman who killed her mother murdered a stranger in the street following a series of police blunders, a watchdog said today.