When finite difference methods are employed in the numerical solution of partial differential equations, the geometry of the region can make the use of overlapping coordinate grid systems desirable. In such cases, unexpected error can result if linear interpolation is employed to exchange information between the grids. The use of such interpolated quantities in the evaluation of finite difference approximations to the derivatives is examined, and various attendant sources of error are discussed. In illustration, the two‐dimensional Laplacian is calculated using data obtained by bilinear interpolation from a neighbouring grid system; the procedure is seen to give unreliable results.