It probably wouldn't be worth the money to
authentically fake a sketch. However someone may
copy one for their own use and down the line it
could get mistaken for the real thing. The sketch
you pointed out is on a cel sketch sheet, meaning
it has registration holes and is of official
size.(in this case movie size.) The paint spots
are the real give away. You could fake those spots
but someone making a sketch for their own use
probably wouldn't bother and someone out to make
money would be wasting their time. Their have been
professional art forgers that have often copied
famous works of art and passed them off as the
real thing only to see if they could get away with
it. They loved fooling people and weren't really
thinking about the money. You never know.