They didn't have to lift a finger.
On Sunday, eagle-eyed fans of the acclaimed drama noticed that the cup appeared during a feast at Winterfell Castle, causing the mistake to go viral on social media within hours.
While the label on the cup is unclear, many fans assumed that it was the recognisable green label of Starbucks – leading many to directly name the coffee giant as they joked about the gaffe on social media.
But while it has since transpired that the cup instead came from a chain in Belfast near the show’s set, it seems that the publicity hasn’t done Starbucks any harm.
According to CNBC, the coffee chain has gained some $2.3 billion in free advertising over the last few days after it was mentioned in the context of the show some 193,000 times across the internet.
Marketing expert Stacy Jones said: “This is a once-in-a-lifetime collision of opportunity for Starbucks.
“But really, this is just the tip of the iceberg, because what isn’t being monitored or estimated is the word of mouth and social media on top of this.”
Although Starbucks were quick to respond to the gaffe, it has since been removed from all future airings of the episode.
Meanwhile, Sunday night’s episode also saw fans responding angrily after Jon Snow headed to King’s Landing without saying a proper goodbye to his beloved direwolf Ghost.