The "one last meal" phenomenon.
This is something that I'm sure many people, including myself, suffer from. Basically, when you're about to start some big diet and exercise routine, you give yourself the excuse to have one last big meal where you go crazy. The idea is kind of like that of a bachelor party. Except some things then happen and you haven't started your diet. A week later, you're about to start your diet again and decide you need to indulge one last time. This goes on and on and the weight gain from those meals will probably be more than whatever you're able to lose on your diet.