I'm forced to disagree with Tez here. Styles like Bjj, Muay Thai, and MMA in general have a lower chance of being taught poorly because there is a standard to adhere to and practitioners of those styles tend to travel around to other gyms to practice with other people.
Can these styles be taught poorly? Sure. However, its going to occur at a much lower rate than a random Karate or Kung Fu school, because they're more than likely training in a vacuum.
Well of course you'd disagree, you are never going to miss a chance to put down a TMA and talk up your BJJ.