Great experiment, and thanks for writing it up. I'd encourage you to get your 10 minutes of internet fame by reproducing the experiment next winter with pictures and measurements. I have to say, despite your experience, I'm still doubtful. Or rather, I'm still almost certain that there must be something besides the temperature that is causing the effect you saw.
For example, let's say you started with the hot water, and then let it cool to 47F--perhaps by putting it outside in below freezing weather. You then ran another tub of hot water, and did the experiment between those two. It definitely seems impossible to me that the new hot one would freeze first, which leads me to believe there must be something measurably different between the cool tap water version and the cooled down hot water.
For example, let's say you started with the hot water, and then let it cool to 47F--perhaps by putting it outside in below freezing weather. You then ran another tub of hot water, and did the experiment between those two. It definitely seems impossible to me that the new hot one would freeze first, which leads me to believe there must be something measurably different between the cool tap water version and the cooled down hot water.