Many cats will eat ribbon, string, rubber bands, hair elastics, thread (with needles attached), pot scrubbers, necklaces and more. These items may be potentially fatal when ingested. If you suspect your cat may have eaten something harmful, contact your veterinarian immediately.
Radiographs (xrays) maybe taken to try to determine if a problem exists and if life saving surgery needs to be performed. Do not wait for symptoms to develop (vomiting, unable to keep food or water down, lack of bowel movements) - time is of the essence!! The longer a "linear" (string like) foreign body is in the intestinal tract, the more damage it can do, potentially slicing through the lining of the intestines and causing a fatal peritonitis.
Avoidance is the key - do not let your cat(s) play with potentially dangerous objects and as with small children, supervise them.