One of the most common mistakes is not keeping good records throughout the year. This can lead to problems when it comes time to file your taxes, as you may not have all of the information you need.    Another common mistake is not taking advantage of all of the deductions and credits that you are entitled to. This can end up costing you money in the long run.   Another mistake is waiting to speak to a professional after year-end. You can only do so much after the fact.   And finally, another mistake people make is not hiring a professional to help them with their taxes. This can be a costly mistake, as professionals can help you to maximize your deductions and ensure that you get the most refund possible.