Revealing mistake: Early on, after Clary blasts the demon with a homemade explosion that blows a huge fireball across the kitchen and blasts out the window, there is no sign of anything being burned, not even the curtains. The kitchen is only in a slight shambles and there are small fires burning in isolated areas that never spread or char anything.
Revealing mistake: When Jace and Clary are on the spiral staircase in the conservatory, the automatic overhead sprinklers go off. Even though the (CGI) water spray can be seen falling all around them and they are not under any cover, they are obviously not getting wet, their hair and clothes staying perfectly dry. When the scene cuts to a different viewpoint, there are some water spots on Jace's back, but they are in an odd pattern and only in a small area, not all over his light-colored shirt, which is dry. As they are walking back to Clary's room, both are still completely dry and the wet spot on Jace's back has disappeared.