This note is an erratum to the article [L. Martin and Y.-H. R. Tsai, Multiscale Model. Simul., 17 (2019), pp. 620--649]. We correct the complexity and speed-up of the proposed algorithm given in section 4.3. ERRATUM Following the above notation, the beginning of the third paragraph in section 5.4 should read as follows.Finally, we verify the implementation of the method in three dimensions. Note that in order to obtain the boundary conditions for each subdomain, we need (M +1) 2 coarse grid nodes on each face of a subdomain. This gives us a total of 3M 2 -3M + 1 shifted coarse grids.