Done that too.
It was almost freezing when we were winterizing. My other half was new to camping, so not wanting to explain everything to him, I tried to do it all quickly because of the cold. I not only forgot to flip the relief valve, I stood in front of the water heater while turning the anode rod. I ended up soaked and with a sore stomach where the anode hit me. I don't think I will forget to do it again like that.