The most common mistake that most would never think of is 'meddling'. The last thing a fish store rep will tell you is to leave your tank alone. Too often people get sucked in to buying useless stuff, constantly redecorate, add/remove things, or simply just eat, sleep, and breath in their tank. Follow what most people say and be patient. If you're gonna do reef, find and research the most natural, low-maintenance system you can find and the rest will take care of itself. Just like in nature, changing conditions will screw up everything.