I think, because eyes are often the last things on a head that you do, you can rush them because you're so eager to get the project finished. I've done that so many times. I've learned now to sit back and take my time. Do an iris, put it down, take a break, maybe pick it up the next day. Obviously if it's a commission you may not get that luxury, but, if it's your own work, take it at your own pace.