We know this from an exclusive excerpt for the book Fire Cannot Kill a Dragon (which comes out Tuesday), and in that excerpt David Benioff and Dan Weiss weighed in on how a coffee cup made an unexpected cameo that slipped past everyone while filming/editing the episode "The Last of the Starks" from the show's final season. For those in need of a reminder, Twitter makes it easy:
"I couldn't believe it," Benioff says in the book. "When we got the email about it the next day, I honestly thought someone was pranking us, because there had been things before where people were like, 'Oh, look at that plane in the background!' and somebody had Photoshopped it in. I thought, 'There's no way there's a coffee cup in there.' Then when I saw it on the TV I was like, 'How did I not see that?'`
Weiss added to that by saying that the obsession with nailing the scene led the obvious to be overlooked.
While modern technology allowed eagle-eyed viewers to shit on the screw-up in real time via memes, it also allowed for the gaffe to be scrubbed from the episode's re-runs. Feel free to note that season 8 had its share of issues.