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As reported by players on reddit and the game's official forums, these 9 cards were being drawn from packs at a much higher rate than other cards. Despite the expansion being comprised of only 6.8% tri-class cards, some players reported drawing 12.5% to as high as 47.6% tri-class cards from the newly-opened packs.
Blizzard responded to complaints that same day on the official Hearthstone website, stating that they were aware of the issue. By 2:30pm PST, about 6.5 hours after the initial statement, Blizzard had rolled out a hotfix for the bug and announced their compensation plan for players that opened any card packs during the affected time. Players would receive 33% of number of Mean Streets of Gadgetzan card packs that they opened during that time (rounded, minimum of 1) for free. This compensation was applied several hours later and all affected players should have now received their card packs. Because the contents of card packs are generated upon opening rather than purchasing, pre-ordered cards packs and packs obtained prior to 2:30pm PST that were not opened were not affected by this bug.