Juicing is all about experimentation and finding a machine, method, and flavor combination that work best for you. That said, these are the common mistakes that I think can easily be avoided.

Making one kind of juice over and over again: Your body wants a variety of enzymes and nutrients so don’t get stuck on one blend.
Adding too much fruit: Because fruits are packed with sugar, I recommend at least a 2:1 ratio to vegetables.
Not drinking it immediately: Fresh juiced is best enjoyed soon after it’s made, as the juice separates and the nutrients begin to break down.
Not washing produce well: Dirt and pesticides will make it right into your cup if produce is not cleaned properly. I like to start with organic ingredients whenever possible.
Replacing a meal with juice: Freshly-made juice is not a meal replacement.
Not listening to your body: Pay attention to quantity of juice, the fruit and vegetable combinations that feel best, and what time of day juicing works best for you.