Continuity errors are inevitable. We're all human, we all make mistakes. Not to mention when you're working on a project set in a different time period, it's hard to go back and forth between worlds.
As you probably already know, Game of Thrones has been dragged all week long after a coffee cup appeared in episode 4. Honestly, it's not that big of a deal. The scene was fairly quick. If you were engrossed in the story, you didn't even notice it. But, for whatever reason people are having a hard time getting over it.