You should not cut your bangs in your skivvies, 5 minutes before your friend is supposed to show up at your house. No matter how close you two are, you will be mortified if she knocks on the door and you're not only undressed, but also only half way through your trim.
You should not cut your bangs while wrinkling your forehead or raising your eyebrows. Especially if you are using your eyebrows as your length guide. Doing so will result in..well, you can probably guess.
You should not cut your bangs all at once, in one big swoop. Start at the center and work your way over to the sides. This way, you won't accidentally cut way. too. much.
You should not cut your bangs and then leave the texturing for later. Even heavy, blunt bangs need a little love from the texturing shears.
You should not cut your bangs and then panic. If you need to, get them wet and re-blowdry them. Now try again and blow them in a different direction. Okay, well.. Look on the bright side: in two weeks they'll be the perfect length.
Good Luck and Happy Trimming!