Usually when stools are pencil thin it's mostly harmless as long as it doesn't occur too often.
If the pencil thin stools only happen occasionally but clear up after a day or so then most likely it's nothing to worry about.
However when the pencil thin stools happen too often or don't clear up soon then it could be a sign of colon problems or even in some cases colon cancer.
I would recommend seeing a doctor to get checked out just to be sure nothing serious is going on.
If it is colon cancer then most times the colon cancer can be cured if it's caught soon enough.
Other causes of thin stools are Irritable Bowel Syndrome, Crohn's disease or even ulcerative colitis.