For years Victoria Police thought they had the DNA profile of Maria James' killer on file, taken from a bloody pillowcase.
But the pillowcase wasn't taken from the scene of the 38-year-old's 1980 murder, in which she was stabbed 68 times.
It was taken from a completely unrelated crime.
Victoria Police admitted on Thursday it had bungled its investigation thanks to a storage mix-up, introducing it to the James file possibly three decades ago.
"This was human error," Assistant Commissioner Steve Fontana told reporters on Thursday.
"Basically, this means we need to go back and re-examine all the exhibits from the Maria James investigation."
That pillowcase was first tested 14 years ago, Mr Fontana said.
It was eventually used to dismiss Catholic priest Anthony Bongiorno, who has since died, as the killer.
All previously ruled out suspects will now be re-examined, but Mr Fontana would not say how many there are.
"We've got to go back and ... see whether we can actually identify whether the offender has left any trace evidence behind," Mr Fontana said.
Ms James' sons Mark and Adam were 13 and 11, respectively, when she died and have long thought Bongiorno was a suspect because he had abused Adam as a child.
Adam James recently detailed his abuse to an ABC podcast looking into the case.
The podcast also looked into the possibility another dead pedophile priest from the same church, Thomas O'Keefe, was involved after Adam revealed he also abused him as a child.
Mark James says he's angry over the latest turn in the investigation.
"I am actually angry. I feel quite indignant," he told the ABC on Thursday.
But despite the disappointment, Mark's also relieved.
"When I was originally told that Father Anthony Bongiorno had been eliminated through some form of DNA-type testing, I found it difficult to accept," he said.
"But now that police have confirmed that Father Bongiorno and others are actually not eliminated, I'm feeling some relief."
Victorian Coroner's Court confirmed it is considering an application to set aside the findings of an earlier, inconclusive inquest.
The case was the first murder that decorated former homicide detective Ron Iddles investigated and when he retired in April, he said it was the one case that stuck with him.
"Here's a lady who could have come out and said Bongiorno has sexually abused my son, O'Keefe has sexually abused my son, I trusted them because I allowed my son to stay there," he told the Trace podcast on Wednesday.
"As I sit here now, if I was to re-investigate I would say that Bongiorno personally is the strongest suspect, based on the evidence of the electrician."