Episode six was accidentally aired by HBO Nordic for an hour in Spain before it realised the error and stopped, according to a post on Reddit.
The airing took place several days before the official premiere.
That proved to be enough for those who leak such material on the Web and plenty of good-quality copies are now circulating on popular torrent sites, according to TorrentFreak.
A second theft, this time in India, occurred on 4 August. The leak, from an HBO distribution partner Star India, has resulted in the arrest of four men. The material was purloined from Star partner Prime Focus Technologies.
Episode six, titled Death is the Enemy, was available both as a single video and as two separate parts after the inadvertent screening. The two-part video was available on the Dailymotion streaming site for a while before it was removed.
TorrentFreak said screenshots and videos suggested that it was made public by HBO unintentionally.
Last December, hackers broke into a computer running Windows 7 at Larson Studies and stole season five of Orange is the New Black.
Photos: courtesy HBO