The Pound-Drever-Hall (PDH) technique is widely used to stabilize the frequency of lasers. Here we report on a routinely underestimated source of error in PDH offset-locking: a shift in the lock point due to the unintended interaction between residual optical sidebands and higher-order spatial modes in misaligned Fabry-Pérot cavities. Significant frequency deviations—up to 50% of the cavity linewidth—can arise when the optical offset is obtained from a sinusoidally driven EOM. We measure this deviation experimenta…