Kyle Smith says Brian Williams' lie wasn't an innocent mistake. I think he's right to look at the direction of the deception:
What Williams' lie was about was what lies are always about: No one who actually scored the winning touchdown on the high-school football team misremembers it as sitting on the bench. The term "fish tale" does not mean you mistakenly tell people you caught a sickly 8-ounce catfish when actually you snagged a 95-pound monster marlin.
It's hard to tell the truth all the time, especially when you can personally benefit from lying just a little bit. If telling the truth were easy we wouldn't value it so highly.
As a society, we're wise to penalize a person who is caught in a single lie. We have to assume that 90% of lies won't be caught, and that a person who is caught is likely to lie more than most people.