Real big mistake, I am sure. You know being able to actually stop without going through tires and ruining your legs is a huge mistake...NOT! I have been running a rear brake on my fixed gear for a long time and have had zero problems (other than normal brake wear issues you get on any bike) and never thought I had ever made a mistake. I still cannot find any mistakes unless I set up the brake like a moron or used improper parts but I wouldn't do that.

There are webpages dedicated to skid patches as if running through tires on purpose because "can't stop won't stop Premium Rush fixie swag" is actually a good thing!