In the Two Towers, when Frodo and Sam are at the Black Gate, you will remember that there is a very large hill in front and a little off to the side of the Black Gate. (the one that Frodo and Sam both end up tumbling down) In the Return of the King, when the armies of Gondor and Rohan are marching on the Black Gate, the entire area in front of the gate is completely flat. There are several wide angle shots of the area, but no sign of this large hill. Where did it go?

Note to slipups.com review staff: yes I know this is technically two different movies, but because they were shot together and meant to be seen together, this should still count as a slip-up for this website's purposes.