Imagine you are flying a big plane with four engines. The engines are steadily humming and you feel comfortable and confident. And now, imagine that one or two engines started to stutter. Though you know that you can still fly on the remaining engines, you become nervous and jittery, and your flight will reflect it.

Your performance is like this plane  - it uses four memories: emotional, visual, motor, and audio to fly confidently through a musical piece. If one of these memories is giving up, you will lose confidence, and your performance will definitely reflect it. That's why it is so important to practice all memories in preparation for a performance.