Don't worry, Serendipity. That 2lb gain is no more than water weight. You'll have been well within your daily calorie intake and you shouldn't give your evening out a second thought (unless it's to reminisce on what a good time you had).
And Jenboo, don't beat yourself up over it. The only thing I would say is that if you found yourself doing the same thing again, you should try to eat a little something because ketosis and alcohol is not the best combination. A little food won't severely affect your weight loss but it helps redress the chemical balance of your blood.