I ate turkey my second Thanksgiving as a vegetarian. I really wish I hadn't. It was the year my step-mom and grandmother didn't make much for me to eat because they and my dad were on the atkins diet at the time (I remember iceberg lettuce, corn, and a dinner roll being my only options--they had the dinner rolls for us kids because they knew we'd want something bread-like). I didn't eat it at home, but later on, we went to a friend's house and I ended up eating it there. I didn't physically get sick from it, but I really beat myself up about it for a while.
Since then, I've learned that if I want to be a vegetarian and not give up my ideals to a hungry tummy, I have to cook for myself. I bring my own food to family parties and holiday dinners now. If I can't bring anything, I always request something for myself (there was a church dinner last year I went to, the entree was rice w/chicken in a cream sauce, so I just asked the sister in charge of cooking if she would just make some plain rice on the side for me and she did--the rest of the meal was fine--there was cheese, bread, fruit, some salad, and a dessert I could eat).
If you're not going somewhere special, make sure you always have a snack with you or that you're not hungry before you go. Eat breakfast, eat lunch, and eat dinner. Plan a snack or two. Don't let tempation be a problem--it's easy to decline meat or another food you don't want if you've already eaten, you can simply say, "Thanks, but I just ate." If it happens again, don't let it bother you too much. Unfortunately, it happens to all of us. Just remember, almost all of the time you're doing your best to help animals by not eating them, which is better than what most people do.