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In brief: The technicalities of the claim come down to minute differences regarding how the game is rendered on screen in MAME versus on an original arcade cabinet. A few months later, Twin Galaxies, an organization that manages video game records, determined that Mitchell's runs were indeed conducted on MAME. While the use of MAME isn't prohibited, misrepresenting scores from MAME as originating on arcade hardware is.
Arcade gaming legend Billy Mitchell is threatening to take legal action against Guinness World Records and Twin Galaxies unless they retract defamatory statements made against him and "immediately restore his achievements and good name."
In February 2018, Donkey Kong Forum moderator and scoreboard maintainer Jeremy "Xelnia" Young presented evidence suggesting some of Mitchell's high scores in Donkey Kong were obtained using an open-source arcade emulator called MAME rather than on original arcade hardware.
As such, Twin Galaxies stripped Mitchell of his Donkey Kong scores as well as all other records of his on the site. A day later, Guinness World Records disqualified Mitchell's Donkey Kong and Pac-Man scores.
Shortly after, Mitchell vowed to conduct a full investigation into the matter to help clear his name. The recent retraction letter from his lawyers includes a 168-page evidence package. Given the massive amount of evidence, it will no doubt take Twin Galaxies and Guinness World Records some time to comb through the findings - perhaps longer than the 14-day deadline Mitchell's lawyers have given.
Mitchell's lawyers claim Twin Galaxies did not provide Mitchell with a fair opportunity to prove his innocence, accepting specific evidence against him but rejecting evidence of equal stature. The firm also believes Twin Galaxies selected a biased third party investigator.
We'll see how this one plays out in the coming weeks.