Much about our tax system is complicated - the forms are complicated and the laws are complicated. Mistakes are made. When do those mistakes rise to the level of tax evasion?
The key is intent. If you use the laws to your advantage, that is tax avoidance. If you make a mistake and inadvertently underreport income or overestimate a deduction, that is considered tax avoidance as well and the IRS will simply demand a correction and, perhaps, a penalty.