In the opening scene, as Matt walks through the graveyard, at least one of the cardboard headstones can be seen flapping in the breeze.
The flapping tombstone was not cardboard. It can be seen to be rigid. It's an error to interpret this an error. This is simply meant to represent the flimsy grave markers left on "Boot Hill". A significant wind can be heard in the background. More than enough to move old loosened grave markers in the wind. This marker doesn't flex like cardboard would. It merely moves with the wind.