The Watchers on the Wall - S4-E9
Revealing mistake: Castle Black defenders drop heavy stones on the free folk and one of them gets hit on the head but forgets to act injured (or dead). (00:23:20)
The Mountain and the Viper - S4-E8
Continuity mistake: When Oberyn first tells Gregor that he will make him confess to raping and killing his sister Elia and her children, he is holding his spear with both hands. In the next shot, he's holding it with just one hand.
Two Swords - S4-E1
Continuity mistake: When Arya and Hound come upon the murdered people Arya is riding facing left. After they pass, she is facing right. (00:48:40)
Answer: This is explained more in the book. Khal Drogo and his bloodriders do intend to help Viserys take the throne, however they do things in their own time. This is what causes Viserys's frustration. In the book, Illyrio Mopatis tells Viserys that he has already waited his entire life to claim The Iron Throne and that waiting a few more years isn't that big of a deal. The Dothraki are a nomadic tribe, which is largely why they have no interest in invading and sacking King's Landing. However, since Khal Drogo eventually grows to love Dany, her assassination attempt by the Westerosi compels him to seek revenge.
Phaneron ★