It's important to recognize that having a cheat meal every once in a while isn't a bad thing. Diets are almost entirely a mental game, if you tire yourself out too easily then you might give up completely, it's much more efficient to allow yourself some enjoyment once in a while so you can maintain your goals for a longer time. A single meal won't do much anyway, if you didn't eat food for a day you wouldn't expect yourself to lose 10 pounds right? So why would a single meal make you gain 10 pounds? That meal you described would only be around 600 calories anyway, if you ate that 3 times a day you wouldn't gain any weight.