One thing that I've only seen happen once, but that really annoyed, is when guests are supposed to bring something to eat, but that something they brought will need to be cooked.
I'm not talking about keeping food in the freezer or microwaving it for a short time, in this case the guest brought something that had to be cooked in the oven. It wasn't my house so I didn't say anything, but I'd be extremely upset if that happened to me, I wouldn't want to spend my electricity/gas cooking when I was expecting the guests to bring the food already cooked!
I'm not talking about keeping food in the freezer or microwaving it for a short time, in this case the guest brought something that had to be cooked in the oven. It wasn't my house so I didn't say anything, but I'd be extremely upset if that happened to me, I wouldn't want to spend my electricity/gas cooking when I was expecting the guests to bring the food already cooked!