Continuity mistake: When Jon and the others emerge onto the other side of the wall, there are 4 people with torches. Pay attention to the one at the back on the screen left. He has his torch in his left hand in this shot, but as the shot changes to the front he now has his torch in his right hand. (00:44:30)
Continuity mistake: When Jon is taking his vow at the tree, the red markings on the tree change between shots. 2 single lines become joined, and a third single line becomes less smudged. (00:45:00)
Continuity mistake: When the group of men emerge at the other side of the wall, the last man with a torch has it held vertically. Camera cuts and now it is held at around a 45 degree angle. (00:44:30)
Continuity mistake: When Jorah gets his royal pardon, the wax seal is hanging freely. Camera changes and now the wax seal is resting on his fingers. (00:31:00)
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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