There were apparently various problems with creating the sandworms for David Lynch's movie version of Dune according to Ed Naha in his book "The Making of Dune".

The Sandworms were small models and the sand was actually fine dust-like spheres. The models were filmed at high speeds so it looked convincing. But this meant that no one saw the results until the rushes.

After viewing the first test footage apparently all the men left the auditorium feeling very inadequate, and everyone agreed they would have to be very careful how they photographed the worms in future.