Green coffee defects are evaluated worldwide as visible abnormalities, physical damage, foreign matter or off-odor, yet the relationship between a grading label and cup quality is neither direct nor uniform. This review integrates international and national classification systems with evidence on defect formation, green-bean composition, roasting chemistry and sensory consequences. It shows that standards differ fundamentally in sample mass, counting basis, legal status and intended purpose, so their values cannot be converted into a common sensory scale. Defects are best understood as disturbances to precursor pools or as processing and safety risks rather than as fixed flavor entities. The strongest chemical evidence concerns black, sour/brown, immature, insect-damaged and aged beans; many other visible categories lack defect-specific odor-active or dose–response data. The term quaker illustrates the central nomenclature problem: standards commonly join it to immature beans, whereas a differentiated physiological position treats emaciated beans as a separate pathway. Rio flavor and potato taste defect provide more specific chemical signatures than most visual defects. A precursor-centered framework is proposed to connect grading with analytical and roasting practice while retaining uncertainty where the evidence chain is incomplete.