I've been cutting and styling my own hair for 6 years. I have 3 clippers 2 corded 1 cordless, and I own a handy mirror with folding sides which can be angled to see the back of my head. (forever alone.jpg)
I've only ever made 1 major error where a damaged guard fell off and I put a grade 0 streak about 2 inches up the back of my head. Instead of lopping it all off, I decided to use boot polish to cover it up. Lasted about 6 hours until someone noticed!