yea it happens, you just have to balance it out eating great the rest of the week. like you mess up one day, one meal even and you feel like a failure, but you just have to remember it takes 3,500 calories to gain (or lose) 1 pound and all you can do is learn from it. I mess up all the time (today i had a slice of pizza but lesson learned from other slips - ONE slice) and if you dont beat yourself up about it too much it will enforce that fact that its not a diet its a *lifestyle change* and are you going to lock yourself in your home and eat just "good food" or are you going to go out on dates/out with friends and family, whatever you do, where you aren't eating "diet food" for the rest of you life are you? Don't let one slip up ruin what you've worked hard for, it happens, i even find it much less frusterating when i see a gain on the scale from a bad day than when i see a 1.5lb gain after days of eating right and exercise
because that definitly happens too... aLOT more than i'd like it to hahaha!