DIA, I would say the most common mistake is having eyes bigger than one's stomach. That means basically that a newbie will sometimes have rudimentary knowledge, on say, summoning (which is a bit too advanced for a newbie, trust me on this) and summon an entity about which they know little or nothing. Before I started casting anything, I basically researched and studied what I wanted to practice. And worked on techniques to make me more grounded, centered and focused enough to feel confident to start casting. The more I cast, the better I have become!
May you only grow!