The community has been debating whether or not a bug was introduced in the most recent Hearthstone patch. Some users have been reporting that the 8.4 patch reset pity timers and caused them to go longer than usual without receiving a Legendary. Blizzard has confirmed the issue today.
Due to the upcoming change alongside Knights of the Frozen Throne, it will no longer be possible to get duplicate legendaries. Blizzard has undoubtedly been preparing for this and as a result of a bug, some players received one fewer Legendary or Epic card.
Community Manager Molly says that the team will be reaching out to everyone who was affected in the next few weeks, mentioning that the team will be creating a "resolution plan."
What are Pity Timers?
Sometime in the past, the Hearthstone community realized that Blizzard's algorithm, which rewards cards when you open packs, are tuned such that as you approach 40 packs in a row without a legendary, the odds become increasingly likely. There is also one for epics.
Many players use tools like PityTracker to see how close they are to a guaranteed legendary. In this case, some started surpassing 40 packs and realized something must have been wrong.