Lord of The Rings: Fellowship of The Ring - Fake Rocks
After the battle with the cave troll, Aragorn rushes over to injured Frodo and brushes past some big rocks which wobble, showing that they are clearly lightweight and not real.
Yes, yet, remember that the cave troll is on his way after them. I admit it is a bad slip-up, yet maybe they were implying the cave troll's weight? I know it sounds corny, but only Peter Jackson really knows.