Suppose you pay more taxes than you owe, either because you neglect to use a tax credit or deduction that you could claim, you put $0 for charitable giving as a result of forgetfulness, poor record-keeping, or wanting to avoid overestimating, etc. The end result is that you end up paying more taxes than what you were required to pay. Is this legal?
Also, what happens if you forget about a small transaction? For example, suppose you make $100K at your job, but also made $25 of taxable income selling your stuff to a friend, but you forget to include the latter under taxable income.