Double-check tile sizes
The single biggest mistake I see people make in tile layout is assuming a 12-inch by 24-inch tile is in fact 12 inches by 24 inches. Most tile is sold in European sizing, and the tile size is 30 centimeters by 60 centimeters. This size could also have a grout joint factored in, so your tile could be closer to 11 3/8 inches. By purchasing a good sample of tiles you can lay them out, check your tiles for warping and soak-test them for quality control.