I have done some searching on the forums and cant really find any info, but someone was talking about getting their beer infected from the gas lines. He was purging his carboy after taking a sample and it hosed the ale. I havent had this problem, but I recently racked my Pliny clone for dry hopping and decided to carb at the same time. I usually carb in the fridge, so I dont think about bacteria in the lines, and I have never replaced or cleaned my gas lines. I have cut off a connector to put a pin lock on, but thats it. Is it possible for your lines to infect your beer when carbing at room temp?
Edit: I have never had any backpressure put beer in my lines either.