Sometimes the changes are so slight it's not noticeable….until you grout and then it shows up. I had it once with a rogue box of tiles. The whole palette had been checked, not by me, apart from 1 box that the details were facing inwards. Customer forgot to mention it to me and they were laid. He did pay me to remove them and relay. It was never noticed until it was grouted and then when you caught it in certain light it showed up, albeit very minimally, but once you know it's there then the game's up.