The Atkins book says that alcohol (straight, not mixed or malt liquor) will not take you out of ketosis, but that as long as the body has alcohol to burn instead of fat, it'll burn that instead. So, it won't make you gain weight, but it will temporarily halt weight loss. So, for that night that you're drinking, you're not burning as much fat and if you're still consuming a lot of carbs those'd probably be getting converted into fat. I'd say the [i]combination[/i] of the alcohol and the McDonald's is what did it.
Next time get the double without the bun. Treat yourself without depriving yourself. Or if you have to cheat, go for the small fry and regular cheeseburger instead.
Good luck, and it's good that you so quickly recognize a falter before it becomes a problem. Don't beat yourself up!