Despite masterminding one of the biggest online games of all-time, World of Warcraft, nothing could have prepared Blizzard for the disaster that was Diablo 3's launch. For a long time, absolutely no one could play the game.
The majority of players were met with the now infamous 'error 37' message, informing them that the servers were too busy, while those who had managed to get into the game were met with something so bogged down beneath players trying to get online that it rendered the game practically unplayable.
Unfortunately, due to Diablo 3's always-online connectivity requirements, even the single-player aspect of the game was locked off. It's almost hard to believe that Blizzard could've so sorely underestimated the amount of people who wanted to play the game - after all, Diablo 3 was a decade in the making, and yet they managed it.
In a way, it was good news for Blizzard - the game sold a ridiculous number of copies, selling through the company's yearly forecast in a single week. Blizzard seem to have learned their lesson as well, as Overwatch - another game that is entirely dependent on being online - launched mostly without a hitch in 2016.