Oxygen isotopic records from corals play an increasing role in paleoclimatic research and climatic modeling. Proxy records of sea-surface temperature are produced by isotopically analyzing samples drilled from aragonitic (CaCO3) coral skeletons. However, drilling aragonite causes a polymorphic inversion to calcite accompanied by 18O enrichment. The isotopic enrichment may occur through atmospheric exchange, CO2 degassing, or fractionation during digestion. Regardless of the mechanism, dry drilling produced isotopic shifts as large as 0.8‰, corresponding to an erroneous temperature decrease of 3 ° C. This error is larger than the global temperature rise since the Industrial Revolution, and a substantial part of the global temperature variation over the past 10 ka. Because the isotopic shift is inconsistent and can occur without the production of measurable calcite, detecting and correcting the isotopic shift may be difficult.