When you're in a rush to wrap up your lab day, it may be temping to save a little time and not label a tube, bottle, or component--after all you know what it is. But that is a sure recipe for disaster.
Establishing good habits at the bench ensures that you don't have to remember all the little details and can focus on the big picture of your experiment. So, label everything during an experiment. Even if it's just a tube of water, and even if you'll only need it for a few minutes.