Don't panic when you overeat or eat something that's not on your plan for the day. Oh yes, you must plan even on the weekend!
However, don't turn your mistake into twice the trouble, fat and calories!!! We often think "well I messed up but I'll start again tomorrow!" NO! Start NOW! Don't simply eat your way into oblivion. One meal does indeed make a difference. As soon as you recognize the mistake..it's time to start again.
Face your mistake, confirm your commitment, and start again NOW!