Planar (D6h) benzene has one (1181i cm-1, b2g) and three (1844i cm-1, b2g; 462i cm-1, e2u) imaginary vibrational frequencies at the MP2/6-311++G(d,p) and MP2/6-311++G levels of theory, respectively! The spurious frequencies correspond to D3d chair (b2g) and C2v boat (e2u) out-of-plane distortions. Numerous, similar examples where planar benzene is not a minimum are documented at the MP2, MP3, and CISD levels with popular Pople-type basis sets, while the RHF, B3LYP, and BLYP methods exhibit no such problems. We show that, in the sp and spd atomic-orbital limits of MP2 theory, benzene is nonplanar. The observed failure of electron correlation methods with unbalanced basis sets to predict planar minima is not unique to benzene but is also found for other π-delocalized molecules, including pyridine, naphthalene, anthracene, the cyclopentadienyl and indenyl anions, and the tropylium cation. Detailed mathematical analysis reveals that an insidious, geometry-dependent, two-electron basis set incompleteness error (BSIE) is responsible for the problem and that balanced, correlation-consistent constructions of basis sets are generally required to ensure reliable predictions for arenes with correlated wave functions.