Think product placement advertising is expensive? What if you could earn your business a cool $3.3 billion in free advertising thanks to a production error on a show that drew in 11.8 million viewers across the globe?

Starbucks Corporation earned itself an estimated $2.3 billion ($AU3.3b) in free advertising over the last few days thanks to a modern-day coffee cup which was spotted during the latest Game of Thrones episode. The truly absurd part? It wasn’t even a Starbucks cup.

Last Sunday night (US time), keen-eyed GoT fans were quick to notice an item that didn’t quite seem to belong amongst the other medieval props.

The rogue cup, which was at first identified as a Starbucks coffee cup, went viral, with photos and videos of the slip-up emerging on social media just hours after the show aired.

Turns out, however, it was not a Starbucks branded cup. It was a basic craft services cup.

Hollywood Branded CEO Stacy Jones revealed the estimated value of all the hype surrounding the ordeal to be a cool $3.3 billion, citing the PR subscription service Critical Mention, which tallied 10,627 mentions of Starbucks and GoT online and on TV and radio across the globe.

Meanwhile, social media analytics and monitoring platform Talkwalker counted over 193K mentions of both Game of Thrones and Starbucks within 48 hours of the show airing, across Twitter, social forums, blogs and news websites.

While the unbranded coffee cup has now been edited out of the episode, the talk about the viral event continues to pervade social media.

Hill Impact CEO Dan Hill told CNBC: “It’s impossible to put a real figure on how much free advertising Starbucks gets out of the situation, but it’s in a totally different category than product placement because it was accidental, which makes it more valuable.”

He added that it’s “too hard to quantify” how much free advertising Starbucks will earn from the bungle, saying it’s simply a “gift that will keep on giving.”

The fact so many people associated the cup with Starbucks is certainly a testament to the global strength of the brand.

Before it was confirmed as a production error, many people questioned whether the placement was intentional or not, especially because the latest instalment of the series took a painstaking two years to film. However, unless HBO decided to change its tune, most people picked it as a rookie accident, mainly because HBO has always been an ad-free, premium subscription service.

Noah Mallin, head of experience, content and sponsorship at media agency Wavemaker said Starbucks’ luck was “lightning in a bottle” and that media or advertising companies would find it challenging trying to reproduce this type of energy organically.