Continuity mistake: After Molly tries the experiment and takes off her safety googles, they reappear and disappear randomly when she hugs Ed.
Revealing mistake: Very obvious copy and pasted dogs (even standing in the same position) whenever there's a general view of a crowd in the main hall.
A Single Piece Was Lost - S1-E10
Visible crew/equipment: When the Gruenaks attack the Scientist a puppeteer is visible to the right of one of the Gruenaks, just before it grabs the bird cage and throws it onto the Scientist's head. (00:28:18)
Revealing mistake: Angelica and the other children are about to crash through the door to attack Lyra and Will. As Lyra runs past a cabinet, a lamp sitting on top is knocked over and crashes to the floor. Dafne Keene, who plays Lyra, reached over and very obviously pushed the lamp to the floor.