Why remains a mystery.
Before November 1 last year, Balclutha woman Judith Tunnah (58) was a caregiver with an unblemished driving record after 35 years of driving.
Yesterday, she sat in the Dunedin District Court in a wheelchair - in which she will likely spend the rest of her life - convicted of careless driving causing injury.
Judge Kevin Phillips ordered Tunnah to pay the 25-year-old victim of the smash more than $10,000 and banned her from driving for six months.
The court heard she was driving south on State Highway 1 at Waihola.
As the road gently bent right, Tunnah crossed the centre line and hit the victim's vehicle head on.
''Both vehicles were totally destroyed,'' a police summary said.
To this day, Tunnah said she had no idea what caused her to veer out of her lane.
Both the defendant and victim had to be cut from the wreckage and were rushed to hospital.
The list of injuries to both women makes for grim reading.
The victim received lacerations to her face that required stitches, a broken wrist, bruised lung, lacerated liver and a heart problem that caused her dizziness and fainting.
She also suffered a broken pelvis, which meant she needed a walking frame for 10 weeks.
Judge Phillips said the psychological trauma was also severe.
The woman was now scared of travelling in cars and when she did, worried constantly that other vehicles would swerve in front of her.
''It's changed, really, forever [her] life,'' the judge said.
Tunnah, meanwhile, sustained a shattered pelvis, which required numerous pins and screws to reconstruct.
Counsel Amanda Short said her client had already undergone seven surgeries since the crash and was scheduled to have a hip replacement in the next few weeks.
There was also a severely lacerated knee with nerve damage, three broken ribs, a bruised lung, fractured vertebrae and broken foot.
Ms Short said the defendant's husband was also on ACC for back problems and the pair had since had to move house to accommodate the wheelchair, costing them $60,000.
Tunnah, she told the court, wished to ''unreservedly apologise'' for the harm she caused the victim.
While she accepted being at fault, she could offer no explanation for her actions.
Tunnah had done voluntary work in the past for the Salvation Army, at times helping people before the court. She never expected to be in the dock herself, Ms Short said.
Judge Phillips acknowledged the defendant's exemplary prior record and accepted there had been extreme repercussions for both women.
He ordered Tunnah pay $7033 for costs incurred by the victim and $3000 for the emotional harm caused.
