A 60-year-old woman passed out and slumped in her wheelchair after being given medicines intended for someone else.
The public health watchdog has blamed a caregiver and a nurse for the error at an unnamed rehabilitation centre in 2014.
Health and Disability Commissioner Anthony Hill said the unnamed caregiver was rushed and stressed and handed over medications intended for another resident, when family members arrived to take their mother to a party.
The 60-year-old, who had many health problems, was given anti-psychotic and anti-spasmodic drugs, rather than her regular drugs.
She turned green, became floppy and tired, and passed out for almost a minute, but recovered later.
Mr Hill faulted the caregiver for not checking the drugs, and a registered nurse for failing to assess the woman fully later, and for failing to seek medical advice.