Here's my experience: one meal won't kill you. I think that what screws people up (at least myself) much more is to think that if you had one high carb meal or even one high carb day, you've blown it and should just give up. If you instead just go back to the program, you'll probably feel minimal impact. I had a little binge once that I was really worried about, but according to my ketostix I was still doing all right. I think what happens the most when people veer off the low carb highway for short periods of time is that the weight loss just stalls for a bit. You probably won't GAIN weight if it was just once. Just go back to it.