Water is the only thing I've ever forgotten that I regretted. I've forgotten just about every other piece of equipment (Tent, Tent poles, knives, cooking utensil, underwear etc, etc, you name it, I'VE forgotten it at some point in time LOL)
But the one time I filled my boda bag and left it in the car accidentally (I hadn't started carrying two extra bottles of water in my pack yet back then). It was a beautiful (Probably around 60 or 65) degree day when I started out, but after close to 2 hours it was up to 80. I reached for my water bag and "CRAP", I cut my hike short right then and turned around and went back to the car. By the time I got back to the car it was over 90 degrees and I was badly dehydrated. That is one mistake I will never make again FOR SURE.