A funny thing happened to me once. I had a headlight go out in my truck, so I replaced it. Two days later, when I went to leave a friends (at 11pm), both of my headlights went out when I turned them on. After searching high and low for three days, I found the problem. Both headlights blew out at the same time. It was time for the old one to go out, and the new one was defective. I replaced them and have had no issues since. From now on I will always replace the headlights in pairs.