Cosby was found guilty in 2018, after a second trial, of sexually assaulting Andrea Constand, a decision based, in part, on the testimony of five other women -- known as prior bad-act witnesses -- who also accused Cosby of sexual misconduct but whose allegations, the defense argued, should never have been presented to the jury.
"Mr. Cosby suffered unquantifiable prejudice where the prior bad-act evidence overwhelmed his second trial," defense attorney Jennifer Bonjean said.
Several justices of Pennsylvania's highest court appeared to agree, questioning whether the testimony of the secondary accusers distracted the jury from the case's core.
"You can still get to the point, with evidence that is terribly probative, that it's just too much, and the defendant did not have a fair trial," Justice Max Baer said.
Assistant Montgomery County District Attorney Adrienne Jappe said the group of prior bad-act witnesses was whittled to five from 19 the prosecution sought to call.
"Why did you need this evidence?" Justice Christine Donohue asked. "You had the complainant, who did testify, and the question in this case was, 'Who were you going to believe?'"
Jappe responded that the other women were needed to bolster Constand's claim she did not consent to Cosby.
"Without this evidence the commonwealth would have had to largely rely on the uncorroborated testimony of Andrea Constand regarding the lack of consent," Jappe said.
The court seemed less impressed with the defense argument that Cosby never should have been tried in the first place because of a non-prosecution deal -- never put in writing -- with a previous district attorney that prompted Cosby to sit for a deposition in a civil lawsuit. In that deposition, Cosby testified about sexual encounters with women to whom he had offered Quaaludes.
"I'm struggling with your argument," Justice Kevin Dougherty said, noting that Cosby, as a celebrity, should have had more resources, savvy and is an "intellectually superior" person who should have known about self-incrimination.
Cosby, 83, has served more than two years of his sentence after he was convicted of drugging and molesting Constand, whom he met through the basketball program at his alma mater, Temple University.
The comedian released a statement on Tuesday that read, in part: "This morning, people around the world witnessed a beautiful presentation by Attorney Jennifer Bonjean regarding two (2) important issues — Immunity & the misuse of a law called, 404 (b) or PBAs (Prior Bad Acts Witnesses). This was not just an historic day for me, Bill Cosby, but it became a beacon of hope for those countless American Citizens of the Keystone State in Pennsylvania Correctional Facilities, whose constitutional rights might have been grossly abused because they lacked resources and means to fight prosecutorial corruption."
ABC News' Linsey Davis contributed to this report.