A particular defect that only occurs in coffees from Burundi and Rwanda and is known as the "potato defect". This is caused by an unknown bacterium that releases a toxin under the skin of the cherry. Although this defect has no negative health effects, it releases an incredibly strong starch/earth/raw potato flavor that masks the other flavors.
Since the "potato defect" is undetectable after processing, the beans must be graded right at the point of origin. The farms are getting better at identifying which beans are defective and can sort them out.