Seven: Gwyneth Paltrow’s decapitated head
What we remember: Detective Somerset opens the box and we see the head of Detective Mill’s wife, Tracy.
What actually happens: We never see the inside of the box.
In fact, only the dialogue and what we can infer of the plot, tells us that her head is inside. But we never see it. To be fair, though, a prosthetic head was made but never used. It showed up in 2011’s Contagion though.