Well, in this case is kind of was. I was too convinced I'd done nothing wrong that I didn't realise that, deep down, I was being a generally lazy and inconsiderate flatmate, not really pulling my weight with chores. I got worked up with excuses, but eventually I gave in and realised I was just plain wrong, and there was no escaping that. So the air is clear, and we're friends again.
I remember having that problem with my flatmate senior year of college. It just turned out to be relationship issues with a girl, wasn't even remotely my fault. It helps to keep in mind a lot of the time a person's disposition isn't really your fault.