One day diet blunder, how bad?

Trying to lose weight is never going to be an easy ride. You will have ups and downs. Don't stress over it, just get on with it. Change it into rewards for being so good the rest of the time. I'm not into counting calories, I enjoy food with moderation. We all have lapses, don't beat yourself up over it. Continue with exercise and healthy eating and little goals along the way with rewards. Have fun and enjoy food and life and exercise.