The launch of Diablo 3 is the case study cited everywhere when speaking against always-online singleplayer games.

The infamous "Error 37" kept untold numbers of players from being able to play a game they'd waited 10 years to buy.

Blizzard said the issue was due to unexpected demand. Speaking at GDC this week, incumbent Diablo game director Josh Mosqueira elaborated.

"The sales team had projected 6.666 million units to be sold in the first year," he said. "We got that in the first couple of days – we just didn’t know what to think, it was mind boggling."

"Even our most outlandish estimates for day one ended up being massively conservative in reality. And it hurt – people had been waiting for ten years to play this game, and the worse fear of an always-online game, is not being able to play. That’s exactly what happened."

Mosqueira said that the game's success was irrelevant in the face of player sentiment. Blizzard "had let them down, and let me tell you - that felt shitty."

Mosqueira, who took over when Diablo game director Jay Wilson stepped down, said Blizzard worked hard to prepare for the launch of the first expansion, Reaper of Souls.

That launch was "smoothest Blizzard launches in history", said Mosqueira.