This is a close relative of fear No. 2. But here, the fear is not of omission, but commission.
This includes things as simple as filing the wrong tax form. It happens. In trying to get through filing as quickly as possible, some folks opt for the easy way out, in this case, the 1040EZ, and end up cheating themselves. Or they choose the incorrect filing status, such as single when they're eligible to file as the more tax-advantageous head of household. Those are just a couple of the many mistakes that tax filers make every year.
The remedy: slow down. You still have plenty of time to do it right. Read the instructions. If you're using software, don't skip steps just to finish. Answer all your tax pro's questions. If he or she says to provide more information, then provide it. A little extra work and attention to detail could cut your tax bill or get you a bigger refund.