Revealing mistake: When Anna and Hans are dancing in front of the lighthouse, we don't see the shadow of the railing or the ground they're standing on.
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: In the scene where Josh Duhamel and Julianne Hough are in the row boat, it starts to rain so they rush to shore. They huddle under a tree and you can see behind them that the rain is only falling right around them - only slightly further away on the lake there are no rain drops hitting the water.