How Kit Harington's Lost Keys Almost Derailed 'Game of Thrones' Season 3

Kit Harington in "Game of Thrones" Season 3. (Helen Sloan/HBO)

For the sake of future seasons of "Game of Thrones," let's hope Kit Harington makes copies of his apartment keys.

Harington, who plays Jon Snow, nearly disrupted production on the third season when he broke his leg last July. He wasn't riding horses or running across icy glaciers, though; he was climbing into his apartment after losing his keys!

"I was an idiot -- 'the invincibility of youth' and all that," he told Entertainment Weekly. "I couldn't even blame it on a film set. And I had avoided skiing for ages because I thought I'd break my ankle!"

[Related: Harington Previews the Arrivals of Ciaran Hinds and a Giant]

With Jon Snow being such a key (forgive the pun) component of the season, producers had to scramble to accommodate his injury. They rearranged other cast members' schedules and pushed back the Iceland shoot. And as Harington's leg continued to heal, they even used doubles to film far-off walking scenes.

Emmy-winning costume designer Michele Clapton pitched in by modifying his footwear. "They asked me to create a boot to cover [his cast], but it was impossible because it was a great big, white foot!" she explained.

When the cast came off, his leg was still healing, Clapton said. "We cut the back out of his [costume] boot so that he could put it on without having to actually step into it and just bound it to his leg. He managed really well, considering the amount of pain he was in. Thankfully, he's much better now, but that was a tricky one."

In the end, fans won't notice Harington's broken leg onscreen, but the injury makes clear just how involved and complicated "Game of Thrones" is to make -- and how one cog out of place could potentially disrupt the hit drama's machine.

[Related: Emilia Clarke and Harington on the Cover of EW]

As Harington said, "We could have been in more trouble had it happened [later in the year]."

The producers were able to figure it all out, since Jon Snow's storyline is so central to the action. In Season 3, the Night's Watch rider meets the Wildlings' leader, "King Beyond the Wall" Mance Raydar (Ciaran Hinds). But going undercover leads Jon to question "whether he does want to be a part of the Night's Watch. He has a lot of turmoil. It's a very tempting way of life for him to join." Hmm, a way of life that includes the beautiful, tough Ygritte?

"Game of Thrones" Season 3 premieres Sunday, 3/31 at 9 PM on HBO.