Revealing mistake: When Elsa is waiting in the lobby, she's wearing a dress which shows her normal, not wooden legs. It's very apparent when she is crying on the floor.
The Last Of The Starks - S8-E4
Revealing mistake: At the beginning when all the dead are burned, we see a close up shot of a dead guy still breathing.
Revealing mistake: In the first shot of the scene where Emma and the others arrive in Storybrooke in their cars, on the left side of the screen, you can see a sign behind a branch. It says "Steveston Museum and Visitor Centre." Steveston is the name of Canadian town which doubles as the town of Storybrooke on Once Upon a Time. (00:07:00)
God Bless Her Rotting Soul - S8-E3
Revealing mistake: If you look at Professor Youens' left foot when he is lying on the couch you will notice a chroma key error - half his foot is almost eaten by the carpet. (00:19:15)
Revealing mistake: On his way to be crowned, you can see the Pope's sneakers under his robe.
Episode #1.7 - S1-E7
Revealing mistake: When the guitarist punches the bass player in the face you can see him miss. (00:50:02)
Revealing mistake: When Tyler stabs Nolan in the arm with the small knife, the knife doesn't have blood on it in later shots. (00:25:05)
Islands in the Stream - S2-E12
Revealing mistake: When George tells Tansy that he's OK if he never ends up with Zoe, then Zoe steps out to agree, her and the boat's shadow can be seen against the backdrop of the sea. (00:35:00)