it seems that it happens with simple geometry props or items.. i mean if the item was modeled otu of props or as a simple mesh with some 3d softwares with poor compatibility export features maybe. i saw a couch once, not made with primitive props, but with nasty lines that appeared on a clean surface. will post a picture of it.. it is "different" from THIs bug but sure cartoon with line behaves in a very specific way that seems to be very strict about 3d modelling :)