Ms Schlef's past history of drug abuse and the fact that she was a heavy smoker put her at higher risk of both aneurism growth and rupture during surgery, an independent neurosurgeon found.
In a tragic twist, the coroner's report detailed how Ms Schlef had initially asked to leave the aneurism untreated, because of the risk of rupture.
Leading endovascular surgeon Dr Ronald Boet said in a statement he had told her of all the risks possible during surgery, including fatality.
Despite the risks, Ms Schlef agreed to undergo the surgery after speaking with Dr Boet and Dr Boet's associate, Dr Andrew Laing, obtained her final consent form.
During the inquest into Ms Schlef's death, Dr Laing - a leading neurointerventional radiologist - admitted one small but disastrous oversight caused the fatal haemorrage.
Dr Laing had inserted a balloon into one of Ms Schlef's arteries to assist with putting a catheter into the aneurism, and accidentally removed it before deflating, causing a brain artery to rupture.
In his explanation to the coroner, Dr Laing said he had not been able to see the balloon was still inflated, a common situation in such procedures.
"The balloon that we have on the imaging screen is actually very difficult to see.
"It is not like a big structure, that is, it's barely visible," he said.
"Pretty much as soon as I started to pull the catheter, I felt resistance ... and realised what I had done and stopped pulling.
"But in that stage, in that literally one second moment, the injury was already caused.
"Once that had happened, there was really no way of reversing the process."
A hospital investigation into Ms Schlef's death cited the absence of a physical barrier or formal memory aid strategy built into the procedure to prevent the inflated balloon meant "a momentary lapse in attention resulted in the inflated balloon being withdrawn".
Ms Schlef's daughter, Nicole Quilter, questioned Dr Laing under cross examination during the inquest as to how a team of experts did not notice the balloon was still inflated, and what changes might be made to prevent similar mistakes from being made in future.
Ms Quilter also questioned why Dr Boet was not personally performing the surgery, which she believed her mother had thought would be the case.
Dr Boet said he at no point had told Ms Schlef the surgery would be a "one man job" and thought he had explained he would be part of a larger team.
The coroner recommended the Canterbury District Health Board continue with research to modify the procedure, recommending a challenge/response system where separate people sanction balloon inflation and deflation and catheter manipulation, rather than have one surgeon responsible for all parts.
The coroner ruled while Dr Boet did not personally carry out the procedure, it was not directly related to either cause or circumstances of the death.
