It was believed that the Se‐rich synthesis condition can suppress the formation of deep‐level donor defect VSe (selenium vacancy) in Sb2Se3 and is thus critical for fabricating high‐efficiency Sb2Se3 solar cells. However, here it is shown that by first‐principles calculations the density of VSe increases unexpectedly to 1016 cm−3 when the Se chemical potential increases, so Se‐rich condition promotes rather than suppresses the formation of VSe. Therefore, high density of VSe is thermodynamically inevitable, no matter under Se‐poor or Se‐rich conditions. This abnormal behavior can be explained by a physical concept “defect‐correlation”, i.e., when donor and acceptor defects compensate each other, all defects become correlated with each other due to the formation energy dependence on Fermi level which is determined by densities of all ionized defects. In quasi‐1D Sb2Se3, there are many defects and the complicated defect‐correlation can give rise to abnormal behaviors, e.g., lowering Fermi level and thus decreasing the formation energy of ionized donor VSe2+ in Se‐rich Sb2Se3. Such behavior exists also in Sb2S3. It explains the recent experiments that the extremely Se‐rich condition causes the efficiency drop of Sb2Se3 solar cells, and demonstrates that the common chemical intuition and defect engineering strategies may be invalid in compensated semiconductors. First‐principles calculations show that the Se‐rich synthesis condition promotes rather than suppresses the formation of selenium vacancy defects in Sb2Se3. To explain such an abnormal behavior, a new physical concept, the defect‐correlation effect, which can be general in compound semiconductors with donor‐acceptor compensation and leads to abnormal defect behavior in Sb2Se3, Sb2S3 and other low‐symmetry or multinary semiconductors, is introduced.