Daenarys' cup of coffee was the first big slip-up that GoT fans noticed and since its the finale, everyone is paying even more attention to the show. There was a small plot twist there: GoT officials confirmed the mistake, but it was actually herbal tea. Not gonna lie, a lot of people are still convinced it was coffee.
Anyway, it appears that human errors were plaguing the series all along. Most of the time, the mistakes were subtle and we didn't realize it since we were so focused on the dialogues or the actions going on. Eagle-eyed fans, however, managed to point out the mistakes throughout the whole series of GoT up to the latest episode. They did not miss a thing and even and, needless to say, the crew had more to learn.
It was easy to make mistakes, however, since the setting is supposed to be a time where everything looks medieval and old. No jeans, no cords, no flying napkins of course. But dragons are ok.
#1 Season 1 Episode 1: This supposedly 'peasant' seems to be a time traveler in jeans and Patagonia jacket.
HBO
#2 Season 5 Episode 10: Dying man is seen to be trying to hide cable cords under his legs.
HBO
#3 Season 2 Episode 8: Tyrion's conversation with his sister Cersei involved his napkin floating from his chest to the table and then back to his armor.
HBO
...could have asked the napkin to wipe his mouth magically.
HBO
#4 Season 6 Episode 9: And Jon Snow's sword looks either flappy or broken.
HBO
#5 Season 5 Episode 6: Nymeria Sand was holding a whip when she was chasing after Myrcella, but then it turned into a dagger the next thing we know.
HBO
#6 Season 7 Episode 6: Jon Snow resurrected and lost one scar on his forehead, meanwhile his chest's scar moved sideways.
HBO
#7 Season 5 Episode 9: Daenarys contracted the greyscale because Jorah touched her with his left hand although he offered his right initially.
HBO
#8 Season 8 Episode 4: The famed, then rumored Starbuck coffee, now herbal tea is just chilling there.
HBO
#9 Season 8 Episode 4: Kings Landing experiencing drought? Possibly. But last we checked, there has been no explanation as to what happened here.
HBO