Consider it: HBO's flagship fantasy drama is one of the most expensive television programs ever made, with a budget of more than $US10 million ($14.3 million) per hour, and each episode is scrutinised and fine tuned down to the last detail.

Which makes the question - what on earth was that Starbucks coffee cup doing in last night's episode? - a more than fair one.

The offending cup makes an appearance early in episode four of the show's final season, a hefty, super-sized episode of around 78 minutes.

It can be found on the table in front of Daenerys Targaryen (Emilia Clarke), which means at least the most pertinent next question - which actor was having a coffee between takes and forgot to hide the cup? - has an answer.

In the scene, set in Winterfell castle's great hall, Tormund Giantsbane (Kristofer Hivju) is toasting Jon Snow (Kit Harington) for his heroism on the battlefield.

Seated behind them is Daenerys, and in front of her, on the banquet table, the offending cup.

Adding insult to injury, Game of Thrones executive producers, writers and showrunners David Benioff and Dan Weiss are actually in the scene, making a cameo appearance as two of the drunken soldiers in Tormund's company.

Expert analysis in the court of social media, where the coffee cup has turning into a flaming Exhibit A, came swiftly: it appears to be a Starbucks coffee. A triplemochaGameofThronesachino maybe?

Compounding the problem is that the glitch comes just a week after the show's cinematographer took aim at critics who said the preceding episode, The Long Night, was too dark for most TV's to display properly.

"If you've got a problem with that, well, you probably need to just turn the lights down, adjust your TV settings, or quit trying to watch it on your phone," cinematographer Fabian Wagner said.

Yeah, well, Fabes, maybe next time while you're lecturing us all about our TV settings you could take a second look at that frame and see if there are any stray coffee cups in it.

Unsurprisingly, the appearance of the cup spread on social media like wildfire.

One fan mocked Daenerys, titling her "Daenerys Targaryen, mother of dragons, the unburnt, the breaker of chains, the drinker of Starbucks".

Another said: "You're telling me they had two years to put together a decent show and they couldn't even spot the goddamn Starbucks cup in Winterfell?"

Echoing a sentiment felt by many, Entertainment Weekly's Jillian Sederholm took the producers to task for their arch tone over the dark cinematography.

"Cannot get over how the makers of Game of Thrones put fans on blast for not having expensive enough TVs to fully appreciate their visually perfect show then this week there's actually enough light to see the screen and their sloppy asses fully left a 2019 coffee cup in the shot," she posted on social media.

The error is reminiscent of one made by the period drama Downton Abbey, in which a modern plastic bottle appeared on the mantlepiece behind the Crawley family.

That mistake, thankfully, was made in a production still and not in a finished episode, though the gaffe and the reaction to it are similar to what Game of Thrones fans will no doubt come to call #coffeegate.

Other historic television gaffes include actor David Duchovny's wedding ring appearing in The X-Files - the actor was married but the character was not - and a much-loved Friends moment where actress Courtney Cox's stand-in is caught in frame while the camera is trained on Lisa Kudrow during a conversation between Monica and Phoebe.

HBO has not responded to the gaffe, though there is no doubt the channel's licensing and branding arm is wishing they could send an invoice to the coffee brand who will inadvertently benefit from the mistake.

- SMH/The Age