I have been there many times when I started calorie counting. I remember going to Subway during the first month of my healthy lifestyle, thinking it was a good option. I ordered a 12" sub with cheese, sauce and the works. It ended up having more calories, fat and sodium than a McDonald's Double Big Mack meal! I still go to Subway but now I know what to order to enjoy a 450 calorie healthy sub. I still find myself shocked when looking up calories for certain foods. I always say to myself, no wonder I was fat

I agree with steph, it's all about planning and trying to figure things out ahead of time when possible. Maybe you could pack a little stash of healthier options in your glove box or purse in case of an emergency.
Don't beat your self up over your little slip. Everyone stumbles sometimes, it's how we learn