Video games like The Witcher 3 are enormous, complicated pieces of machinery with thousands upon thousands of moving parts. Make one mistake and you can break everything—or, as one developer discovered, you can accidentally unlock every door in the game.

In October, I visited CD Projekt Red’s offices in Warsaw, Poland, for my book on the stories behind how games are made (which has a chapter on The Witcher 3). While interviewing the developers there, I heard a few great anecdotes that didn’t make it into the book. Here’s one of them.

Think of this as free DLC.

One day, during the development of The Witcher 3's second expansion, Blood and Wine, the designers at CD Projekt Red realized that something was horribly wrong: They could walk into any house in the game—even the ones that were meant to be part of the scenery. “We couldn’t figure out for a long time why it was happening,” said Miles Tost, a level designer on The Witcher 3. “It was terrible, because some buildings wouldn’t have any terrain underneath them and when you walked in, you’d basically just fall into nothingness. For optimization purposes you don’t put content into every single house, only the ones you can actually access.”

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Tost and the rest of the design team scoured the game’s world, trying to figure out how this had happened. Doors in The Witcher 3 weren’t automatically attached to buildings—instead, the designers had to place each one manually—so going through them all was a tedious process. Some doors were meant to be always open, while others were meant to be locked with specific keys that you could find in the world. Others, of course, were never supposed to open at all.

Eventually, the developers of The Witcher 3 identified the problem. During a quest in Blood and Wine involving the siege of a castle called Dun Tynne, CD Projekt Red’s quest design team had decided that they didn’t want the player to be able to enter any of the buildings. This was meant to be a linear, streamlined mission—Geralt shouldn’t be getting distracted. The Witcher 3 was supposed to lock all of the doors for the duration of the quest, and then, once the quest was over, unlock those doors once again.

Problem was, as Miles Tost recalled, the game had no way of knowing which doors had been open before the quest and which ones had been locked. So it would just open everything. “This would result in all the doors in the game being unlocked,” Tost said. “And I remember the solution for this was quite bitter—the quest designer had to actually go through every single door in the game and add this tag. ‘This is a door that was closed before, and it should be closed again after.’”

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It’s just one example of how a small mistake can be catastrophic for a game’s development, and one of many, many reasons that making games can be such a long, brutal process. “These things happen, and they’re tiny changes that you don’t anticipate, and they result in these huge huge problems,” said Tost. “You don’t really think about how something so small can actually fuck up everything.”