Continuity mistake: When the Faith Militant descend on Ser Loras Tyrell's training session there is a shot after the Militants have come down the stair of Loras fighting from behind a man's shoulder, in this shot you can see they are fighting on grass surrounded by walls with sheer drops with no entry point other than the stairs the militants walk down. In the next shot you can see the 9 militants that have come down the stairs approach Loras from one side, but suddenly 6 militants join him from the opposite side, where they definitely were not before. In the zoomed out shot you can see there is nowhere for these men to have come from. (00:11:15 - 00:11:50)
Continuity mistake: Towards the end there is a body of a dead Son of the Harpy lying between the fallen Unsullied and the side corridor visible in the wall, located very close to either. In the next show the body is gone. Also, as the camera angle changes, the blood stains on the walls look differently. (00:48:50)
Chosen answer: Considering the show was in production for nearly a decade, it is hardly surprising that many characters were recast. Actors constantly move on to other jobs, creating scheduling conflicts. Some may not want to be tied to one project for so long a period, particularly for relatively minor roles that paid little money. As some characters became more prominent, the producers did recast a few roles with better-known actors. For example, Max Von Sydow replaced the actor who originally played the Three-eyed Raven. Some actors voluntarily left for personal reasons. Jennifer Ehle was originally cast to play Catelyn Stark but dropped out because she'd recently given birth. Other actors claimed they were replaced for "political" reasons. Here is a website with a rundown of which actors were replaced and why: https://screenrant.com/game-thrones-characters-recast-new-actors-every-why/.
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