We had a bathroom retiled in Puerto Rico a few years back, and the craftmansship was amazing. But on the final inspection I noticed that a tile on the shower wall was upside down. When I pointed it out to the tiler, he smiled and told me (loose translation) that while he aimed for perfection, he always put in a deliberate error so as to not get too cocky! I loved it, and think about it whenever I make mistakes - the little imperfections show that it was made by loving, human hands!