One of those days

Do you ever have one of those days where everything is going ok, not great, but better than the day before which was hellacious. Which is a good thing, or should be a good thing. Then suddenly, one little thing happens, and it's like *snap* and now your not ok at all. That's the kind of day I had today. And it was one little thing, nothing major, everything should still be fine, but in your mind it's not? I hate days like that. I'm just so frustrated.