However, this time, the source of the leak was not hackers or thieves, but rather the show being mistakenly released on to its broadcaster's streaming platform too early.
The episode, titled Death is the Enemy, has since been withdrawn, but not before it was copied.
Twin Peaks suffered a similar error recently.
A spokesman for HBO Europe blamed an unnamed contractor for the most recent blunder.
"We have learned that the upcoming episode of Game of Thrones was accidentally posted for a brief time on the HBO Nordic and HBO Espana platforms," Tom Krogsgaard Nielsen told the BBC.
"The error appears to have originated with a third-party vendor, and the episode was removed as soon as it was recognised.
"This is not connected to the recent cyber-incident at HBO in the US."
Reports indicate that high-definition footage from the episode was initially posted to YouTube, the Daily Motion, Instagram and other social media sites.
Although it has since been removed, different copies of the complete 66-minute show are still being shared on several file-sharing platforms, according to the news site Torrentfreak.
HBO has suffered a series of security setbacks over recent weeks.
Unidentified hackers have released Game of Thrones scripts, company documents and unbroadcast video episodes of some of HBO's other shows, including Curb Your Enthusiasm and Insecure.
They claim to have stolen 1.5TB of data in total.
In a separate incident, pirated copies of the 6 August episode of Game of Thrones were spotted online several days earlier in the week, featuring an Indian broadcaster's logo.
Four people have since been arrested, three of whom work for a Mumbai-based company that stored and processed the show for an online streaming service.
Not what it seemed
HBO is not the only broadcaster to have been caught out by its own technology.
Sky Deutschland accidentally uploaded episode 14 of Twin Peaks: The Return to its streaming platform on 7 August, instead of episode 13.
It blamed "wrong labelling of the broadcasting file" for the error, which is somewhat ironic as one of the themes of the series is mistaken identities.
Although some viewers shared spoilers and screengrabs to Reddit, reports at the time suggested the episode had not been pirated as a consequence.
Photo by: HBO Caption: HBO’s Spanish and Nordic offshoots streamed the episode early