Some people are very lazy when it comes to observing rotaions. I myself work for Discount Tire Company. A directional tread tire mounted backwards will only affect driving in wet conditions. It is simply just the water channels that are directional. Some people say that it is the threads as well, but that is just a rumor. Always make sure your tires dont have bludges in the sidewalls. This can result in a blowout and could ruin your pretty little car
An imprint in the sidewall is nothing, it is simply where the threads are connected, but a bludge means the threads have broken and the only thing holding the air in is the thin wall of rubber (which is severly damaged on the inside when the threads break).