Lumps happen if the plastic is chilled by a poor quality mold before the plastic has had time to stretch and be evenly formed. Above is a photo of one of the lumps of a refrigerator door shelf. ( General Electric - model L15YPRW-1 Ser. AH312899V) -purchased in 1992. The lumps are thick and strong but the the plastic of the surrounding area is considerably thinner and weaker.