Dark Wings, Dark Words - S3-E2
Continuity mistake: When Brienne of Tarth is facing off against Jaime Lannister on the bridge, in a shot from behind the blade is angled upwards, not touching her arm, then in the immediate next shot from the front the blade is now lying along her arm.
Continuity mistake: At this point, Jamie Lannister has lost a hand at the, well, hands of his captors. As they shove him to his knees in front of Lord Bolton, he uses both of his hands, with no stump being visible. (00:12:39)
Continuity mistake: When the camera pans out from Jon and Ygritte standing on the wall kissing, Tormund and Orell, who were sitting next to them, are nowhere to be seen.
The Bear and the Maiden Fair - S3-E7
Continuity mistake: Jamie is getting his stump tended to and some white ointment is rubbed onto it before it is rebandaged. Before the bandage is placed however, the ointment disappears.
The Rains of Castamere - S3-E9
Continuity mistake: At the climax of the Red Wedding massacre, Robb crawls towards his dead wife Talisa from the direction of the doors, which are at the opposite side from Walder Frey's throne. Robb turns Talisa onto her back from her left side (ie. he is on the right hand side of her body), and looks down at her face. This means that Talisa is lying in such a way that, if facing Walder's throne, her head is on the right and her feet on the left, towards where Catelyn is holding Walder's wife hostage and pleading for Robb's life. However, when Walder refuses, Robb has changed position and is now on the left side of Talisa's body, which is presumed to be facing the same direction as before as there is no reason for it to have changed. Robb then turns towards Catelyn who is still on the left side of the room (if facing from The Door), and is stabbed by Roose Bolton. As Robb collapses to his death, looking directly at Catelyn, he lands next to Talisa, whose head is now closest to Catelyn with her feet facing the other way, meaning that her body has somehow rotated by 180° in a matter of seconds. This is an especially bad mistake considering that several people have claimed the Red Wedding was completed in one take, which is not possible if this error exists unless Oona Chaplin for some reason decided out of boredom to play spin the bottle with her own body while out of shot, which would have interfered with Richard Madden's position anyway. (00:47:00 - 00:50:00)
Answer: Daenerys and her allies don't just want to kill Cersei, they want to claim King's Landing and free her people from Cersei's grip. The problem is the people of Essos viewed Daenerys as a liberator but the people of Westeros view her as an outsider and usurper. They would never follow Daenerys if she had Cersei assassinated. That is Daenerys' dilemma, she certainly has the ability to wipe Cersei out and obliterate her armies but doing so would make her a tyrant. Which as it turns out is exactly what happens.
BaconIsMyBFF
But no-one has to know that Cersei was assassinated. Arya has the ability to impersonate anyone she kills, so she could pretend to be Cersei afterwards and profess to the citizens of King's Landing that she has yielded the throne to Danaerys and that she is going into exile.
Phaneron ★
That plan would be incredibly suspicious. Knowing what they know of Cersei it is highly unlikely the people of King's Landing would believe that she would accept defeat so easily and then voluntarily exile herself, never to be heard from again. In order for that to work, all of Cersei's advisers and closest allies would have to be similarly eliminated, or they would have to be on board with the exile plan. If they are all killed it sort of makes it obvious that something is amiss. There's no way they would be fooled by Cersei suddenly doing a 180 and completely changing her personality by accepting defeat without a fight. If any part of this plan goes wrong then Daenerys would look worse than just an assassin, she would also be deceitful to the people she hopes will willingly accept her rule.
BaconIsMyBFF