. . . when a bolt falls out of one of your machines? Its just laying there on the floor, you don't know how long its been there or where it came from. Could be this machine or that, you don't know because you could have kicked it, moving it away from whence it came. You examine the nearest machines exhaustively . . . but no missing bolts are to be found. So, for the next month you look at that bolt and worry about which machine is going to fall apart. Shops are full of nasty surprises.