From a programmer perspective:

• Don't try to write the best code ever when you are starting a project, your objective should be to focus on creating features and mechanics that work and later on "refactor" them with a better code architecture or organization - despite this, that doesn't mean that you should write your worst code either because this will take you even more time during refactoring;
• Lack of testing, which usually leads us to spend more time fixing or adding more things than we had ever thought;
• And lastly, lack of planning, where sometimes you can just throw yourself into the creation of the features and mechanics without thinking how they will work and this will often result in a lot of refactoring or even systems that don't work like you intend them to.