Yet ANOTHER Two Towers Page

The gash on Merry's face changes sides throughout the movie. Sometimes it is on his left brow, sometimes on the right.

When Treebeard is talking to Merry and Pippin at the Entmoot, the stars that can be seen behind him change.

Merry and Pippin switch position between shots when Merry is making his speech to Treebeard at the Entmoot.

When Grima Wormtongue is talking with Eowyn, he puts his hand on her face, but when she says, "Your words are poison," we see that they are several feet apart.
(Now even I noticed this one)

As women and children enter Helm's Deep's caves, a rock stalactite swings back and forth after an extra touches it.

In the shot where Aragorn orders the men of Rohan to fire their arrows (just after the elves do from the wall at Helms Deep) his sword's blade disappears as in comes toward the camera.