Going to go ahead and add some of the most common mistakes I've seen in the last ten years that aren't included here:

1. People start too big and fail to scope because they have no knowledge of the wide variety of tasks involved in making a game. (My advice: Start extremely small and have an experienced mentor help you out to begin with.)
2. People start programming on day one and putting things together immediately without thinking about anything. (My advice: Start with pen and paper prototyping and roleplaying/testing your game idea.)
3. People think a good game will sell itself and think about marketing/reaching their audience way too late. (Start building your game and its community as early as possible.)
4. People think programming is all that's necessary to be a game developer. (There are several skill sets necessary to make games. They're dang tough to make.)
5. People start playtesting way too late. (Start as soon as you have a broken build. Testers very quickly dispel a lot of assumptions you've been making about your game.)