A twig on Jane Rimmer's body was contaminated with female DNA from an unrelated crime, the Claremont serial killings trial has heard.
Bradley Robert Edwards, 51, is on trial in the West Australian Supreme Court accused of murdering Sarah Spiers, 18, Ms Rimmer, 23, and Ciara Glennon, 27, in 1996 and 1997.
PathWest scientist Scott Egan testified a sample from the twig, known as exhibit RH21, was examined by scientist Agnes Thompson in 2002, just days after another scientist examined an item for an unrelated crime.
The court heard DNA from that other case made its way onto the twig, which Mr Egan put down to a "manual handling issue" because the same set of tubes were used for both cases.
"There might have been an operator error," he said on Friday.
This type of contamination is called secondary transfer, which is what defence counsel Paul Yovich claims is most likely to have occurred for Edwards' DNA to end up on Ms Glennon's fingernails.
The court previously heard there was a two-week gap between lab testing on the fingernail sample and a swab taken from a rape victim, which had Edwards' DNA on it.
Edwards, a former Telstra technician, admits raping a 17-year-old girl he abducted and dragged through Karrakatta cemetery in 1995 and attacking an 18-year-old woman as she slept in her Huntingdale home in 1988.
After the Huntingdale attack, Edwards left behind a silk kimono, which also had his DNA on it.
But it was his fingerprints left behind at the home, which helped tie all the loose strings together for police to put an identification on the DNA collected.
Edwards' fingerprints were already in the database because he was convicted of attacking a woman from behind at Hollywood Hospital in 1990.
The court also heard on Friday that former PathWest lead scientist Laurie Webb tested a swab taken from Ms Rimmer, which UK lab Cellmark discovered in 2017 had his DNA profile on it.
Ms Rimmer's fingernails were also found to be contaminated with Mr Webb's DNA when tested by Cellmark, even though he was not directly involved in the examination or testing.
Mr Webb was sacked from PathWest in 2016 for breaching testing protocols.
Another branch from Ms Rimmer's dumping site had the DNA profiles of two PathWest scientists - Aleksander Bagdonavicius, who examined it, and Louise King, who tested it.
Cellmark also found Mr Egan's DNA profile on a sample taken from Mr Glennon, which he said was possible because in 1997 he would not have worn a face mask.
The decomposed bodies of Ms Rimmer and Ms Glennon were found dumped in bushland at opposite ends of Perth, but Ms Spiers' body has never been found.
The six-month trial, which has completed day 54, will resume next week.