I left my sleeping bag.
I was just camping near the coast at Laguna Seca in Monterey. How cold could it get right?
I brought one of those fleece sleeping bag liners, because that is all I could fit in my backpack. I figured it would be enough. It wasn't. I ended up sleeping in someone's truck, and it was still cold. After the third night, while packing up to go home, one of my friends hears me talking about how cold it was without a bag. He pulls out a big fluffy sleeping bag from his car, and says "you should have said something, I brought an extra."