Revealing mistake: Both Sersi and the student she saved have been covered by the fine dust she turned a fossil into, but the only trace of it is a little dust on top of the girl's hair; there's not a speck on her uniform or anywhere else. (00:09:55)
Revealing mistake: When inside the Forge Arishem says "I created the deviants, Sersi", you can spot Gemma Chan's beauty mark changing side - the shot is flipped. (01:01:15)
Revealing mistake: During the attack on Druig's village, there's a shot of Gilgamesh and Thena running towards the camera, then stopping as two Deviants fly over them. During this scene, you can see Don Lee (Gilgamesh) briefly look straight at the camera (he's not looking at either the village or the Deviants, since they headed in the opposite direction). (01:13:50)
Revealing mistake: At the beginning of the scene with Thena in the cave, she changes the weapons in her hand to two swords. It cuts to a closer angle, then another, and she (deliberately, not a mistake) switches one weapon to a curved blade. Cut to a wider angle, and someone forgot to add the CGI for that blade, because Angelina Jolie's left hand is gripping thin air there. (02:01:55)